After many unsuccessful attempts to train children from birth in martial arts; to make them inculcate it like a native language (the most successful being The Mad Dog), David Cain, then a member of Ra's League of Assassins, decided that the answer lay in the child’s genetics to create this "One Who Is All". With Ra's permission Cain searched for the perfect mother for this child, finding her in the Wu-San sisters, Sandra and Carolyn, of Detroit, who practiced martial arts with each other nearly every moment of their childhood in a type of ‘sister language’.
Cain sympathized with the younger sister, Sandra; having noticed that she held back out of love for Carolyn. To "help" Sandra reach her full potential, Cain murdered Carolyn, then, when she embarked to hunt down her sister’s killer, he lured Sandra into an ambush by the League of Assassins where he defeated her. Cain spared Sandra from death on the condition that she bear his child and leave it for him to raise.
Awed by the potential heights she could reach, Sandra agreed to Cain's bargain; and would later go on to become Lady Shiva: creator and destroyer, the greatest martial artist in the world. Her deal with Cain had an ulterior motive however as Sandra's hope for her child was that it might one day grow to be the one force that could stop her reign as Shiva.
Trained by her father to be the ultimate weapon Cassandra was not taught to speak or read. Instead the parts of her brain normally used for speech were trained so that she could instead read other people's movements and body language and by extension predict, with uncanny accuracy, their next move much like how a person might predict how a sentence would be structured by watching it being created. This also caused her brain to develop learning functions different from most, a form of dyslexia that hampered her future ability to read and write.
Training her in a variety of secluded locations with a variety of training opponents, Cain recorded the sessions for unknown reasons. When his daughter was 5, Cain began taking Cassandra along on his hits. At age 8 he decided she had progressed far enough to test her abilities in the real world. He took her to a hit; the target Faizul, for her to perform herself. At the time Cassandra had no idea what she was doing and believed it was only a game however after striking a deathblow, she "read" the target as he died and was scarred by the event.
In addition to scarring her emotionally the experience led her to realize that murder was wrong and she ran away from her father.
Cassandra spent the next nine years homeless as a wandering drifter.
At the age of 17, Cassandra entered Gotham City on the eve of No Man’s Land as a way to further hide from her father. Eventually, she became acquainted with Barbara Gordon and came to be one of her most able and trusted agents; sending and receiving information from within the city, despite her dyslexia. After saving James Gordon's life from an assassination attempt by David Cain (who had been hired by Two Face) and further halting the contract by confronting the contractor; Two Face, on her own initiative, she was given the Batgirl mantle by Batman (Bruce Wayne) with added support from Barbara. She became Barbara Gordon's ward and, in a sense, the Batman's adoptive daughter; both looking out for her in their own way.
As the No Man's Land continued Cassandra was introduced to the rest of the Bat Family with varying levels of responses (Azrael (Jean Paul Vallery) liked her, Robin (Timothy Drake) was intimidated by her, Nightwing (Richard Grayson) with-held judgement, although he was sceptical). Usually going out under the Batman's guidance, Cassandra was finally allowed to go on a solo mission when a petrol station needed guarding from a local gang. By the end of No Man's Land the Batman had noticed her death wish, a result of her emotional scarring and the self-conception that she must ‘pay’ for her crime, and forced her to make a decision between suicidal action and self preservation before she would be allowed to continue as Batgirl. She opted for self preservation.
Despite her notable handicap of being unable to talk, read or write, Cassandra did remarkably well in her position as Batgirl. Capable of understanding others' intentions due to her ability to read body language she was a more than capable listener, even if she couldn't reply. It was this trait that made her one of the Batman's most loyal and trusted followers and helped her to survive when the Bat Family became hunted by the governmentally sanctioned Blood Hawks.
Although the fact that she couldn't talk frustrated Barbara Gordon, she proved herself useful in other ways during a team up with the spectral vigilante Ghost in hunting down disappearing corpses. The running successes and final understanding of her mission as Batgirl allowed Batman to give her the privilege of running solo in Gotham City on the condition that she faced no 'costumed criminals' (Joker, Bane, Two Face, Penguin, etc).
From going undercover in Madras with Batman to saving heroic civilians from the mob, Cassandra continued to impress, especially when, despite Batman's warnings, she ended up face to face against a meta-human during a mission and won. She even managed to, despite her restriction to Gotham City, make it on the reserve member list of Young Justice. In the background however a rift between Barbara and Batman was slowly emerging, and at the same time the growing image of Batman as a father figure was beginning to seed in Cassandra's eyes.
Batman eventually learned about Cassandra's past when David Cain transmitted a tape he had made of Faizul's death to the Batcave, but despite this Batman continued to accept Cassandra as Batgirl. However, when a run-away psychic altered Cassandra’s mind so that she could understand words, she lost her ability to read body language and by extension lost her ability to fight effectively. After she almost died trying to protect the psychic from assassins Batman took away her costume until she could better defend herself; taking the responsibility of retraining her upon himself. The training remained ineffective however, perhaps due to Cassandra's dwelling on her lost abilities instead of focusing on accumulating more, and Cassandra’s wish to roam the streets with or without the costume.
It was during one of these unauthorized vigilante outings that she first encountered Lady Shiva, unaware that she was her mother. Shiva noted Cassandra's missing skills and offered to retrain her but in exchange they would fight again in one year; a death duel.
Cassandra began training immediately; going after every talented fighter she came across in order to gain more skills, she even (hearing about his notoriety within the Bat Family) freed the Joker then apprehended him again in order to prove herself.
Batman and Oracle deduced that Cassandra's death wish had not gone away and that it was due to guilt over her murdering Faizul. They began looking into what had brought the death wish back. A further jaunt with Kon-El that nearly ended in both of their deaths only increased the need to discover the origins of the death wish.
Her reckless training was interrupted when an Officer Down was called over the frequencies and the Bat Family became involved in finding James Gordon’s assailant.
It was during yet another training session in the Batcave that Cassandra first met her future friend Stephanie Brown and it wasn't long until Batman gave her a cave of her own to operate from. Her training crusade continued as she found even more powerful opponents like Bizzaro Supergirl and Shadow Thief during the Joker's Last Laugh as well as numerous female villains during the aftermath of Our Worlds at War. These activities had not gone unnoticed by Batman and Oracle, having finally deduced the death duel with Lady Shive but despite Babara's protests Batman allowed Cassandra to continue until the fateful day arrived and Cassandra fought Shiva on the building tops of Gotham City. Shiva won the battle fairly easily, killing Cassandra with a shot to a nerve cluster near the heart but, within a temple devoted to her, Shiva brought Cassandra back to life and requested another fight. This time, with her death wish satisfied, Cassandra Cain won and became the first person to defeat Lady Shiva in single martial combat.
Unfortunately the victory would be short lived as Bruce Wayne was convicted under suspicion of murder and Cassandra arrived back to the cave in time to not only learn that Wayne had escaped but to see Batman leave the Bat Family. It was during an investigation into the murder in question that Cassandra discovered that the Batman and Bruce Wayne was the same person.
Loyal to the belief that her father figure was innocent she, along with Nightwing, discovered critical clues that pointed to Bruce Wayne's innocence and, eventually, incriminating evidence toward the real culprit; David Cain, who had initially been hired to frame Bruce Wayne for murder by Lex Luthor but, upon discovering the Bruce was Batman, opted instead to drive him away from the Bat Family and by extension, Cassandra.
Her father safely behind bars and no battle-to-the-death with Shiva to train for, Cassandra focused on her crime fighting career, assisting in the defeat of The Network; a group of criminals formed solely to destroy Wayne Enterprises. Cassandra's lessons in detective work began soon there-after, helping to track down Alpha, a rogue agent that had temporary amnesia. The discovery that David Cain was her true father and not, as she had believed, the man who had kidnapped her from her real parents made Cain's crimes more personal than they had before and with the added weight of Stephanie Brown's dismissal as Spoiler from the Bat Family Cassandra began to question what it meant to be part of a family.
After surviving near execution by General Zod and later encountering Black Wind, a terrorist leader of the Sakuri Lions, Cassandra closes up. Barbara Gordon summarizes that she needed some time off and forces her to go on a cruise ship, organized by Batman. But in reality a holiday was not what Cassandra was interested in, she was instead interested, or becoming interested in, relationships. The cruise ship had turned out to be a suspected conflict zone between the Sakuri Lions and Namali; a corrupt oil tycoon, and, thanks to Batman's manoeuvrings; Cassandra was in the perfect place to prevent it. What was not counted on though was the presence of Superboy (Kon-El) and the ruminations of the two interacting with each other. Cassandra attempted to pursue a relationship with Kon-El, going as far as going to Smallville to visit him but, when she realized that her attraction was not due to love but due to loneliness, she settled at being friends instead.
To compensate for her loneliness she started to attend parties along with her fellow heroes in an effort to become part of the community. But with the arrival of Doctor Death, the death of Black Wind and the harsh and restrictive attitude of Batman she became more frustrated with her position as a lonely 'Dark Knight'. Her frustrations came to a head while preventing slave marketers from obtaining Gotham citizens and, with the help of Dr Death's Soul drug, she became embroiled in a heated fight against Batman. In the end the fight ended peacefully, despite the destruction of the Sprang Bridge, and their father-daughter relationship strengthened even more.
With this new respect came added trust as Batman began to leave Cassandra alone to take care of Gotham, resulting in her single handily stopping Poison Ivy (Pamela Isley), and being given more responsibilities such as being taught to fly the Batplanes and Batwings. Due to this she was prepared to take over for Batman during his breakdown in the City of Light fiasco.
With the forced retirement of Timothy Drake from Robin, Stephanie Brown was allowed to take the mantle while Cassandra was allowed to become part of the Justice League Elite; albeit as a sleeper agent for Batman. However, Stephanie failed her probation as Robin and was fired again which resulted in her attempting to prove herself, resulting in a city-wide gang war across Gotham and Stephanie's suspected death. Batman thought it wise that, due to the Gotham City Police Department's new intolerance against the Bat Family, that Cassandra follow a newly reinstated Timothy Drake to Bludhaven.
With Bruce Wayne's financial support, Cassandra quickly makes a name for herself in Bludhaven by defeating the Society of Evil and constantly disrupting the Penguin’s schemes. This resulted in the Penguin hiring Deathstroke (Slade Wilson) to kill her, who decided to let his daughter, the Ravager (Rose Wilson), do the job instead. Cassandra defeated the two by critically wounding Rose, giving Deathstroke no choice but to get her medical attention.
It was during fathers' day that Cassandra became infatuated with knowing who her mother was. Suspecting Lady Shiva (Sandra Wusan), she began interrogating all the contacts she had (including Batman and her father; David Cain) she eventually found Lady Shiva in Nyssa Raatko's sanctum training the new League of Assassins. She also met her 'brother' The Mad Dog. Although she prevented Nyssa's involvement in the Infinite Crisis, Cassandra was killed while preventing her 'brother' from killing other League members. Shiva resurrected her daughter however, asking only that Cassandra stop her by killing her. Cassandra obliged; braking her mother's neck and leaving her hanging over a Lazarus Pit as she left to return to Gotham.
Returning to Gotham City, Cassandra discovered that not only had her adoptive family left on a year-long training trip without her but that Batman had left a recently cured Two-Face to protect Gotham in his stead. Still, Cassandra still retained the Batgirl persona and patrolled Gotham while staying at Wayne Manor under Alfred's care.
Due to Alfred's influence and due to her attending English classes between her nightly activities, Cassandra would gradually learn to read and write. It was during one of her patrols however that Deathstroke, who had been planting seeds of doubt in her mind for some time over how much her adoptive family actually cared about her, managed to not only inject her with a controlled dose of his serum but influence her enough to join him in a plan to form an army involving David Cain's 'daughters'.
Under Slade's influence Cassandra gained control of a part of the League of Assassins, assassinating Nyssa al Gul in the process, and became a member of his Titans East. As leader of the League of Assassins she also attempted to kill Kara Zor-el in an arranged hit against her and masterminded the escape of David Cain from prison; attempting to frame Timothy Drake for murder and lure him away from Batman in the process.
She would eventually be freed from the serum's influence when Timothy Drake injected her with an antiserum during a confrontation between the Titans East and the Teen Titans, however, suffering residual effects of the drug, she briefly continued to work for her father in attaining the drug Pheno before fully coming to her senses and returning to the Bat Family.
Feeling partially responsible for what happened to her Batman allowed her to return after a strict detoxification program and had her under strict curfew. Nightwing, however, was not so accepting, regarding her with open hostility and suspicion. When Cassandra began seeking vengeance upon her father and Slade for drugging her, she had to do it secretly due to their mistrust but eventually she was found out, resulting in her clashing with Nightwing and stealing a Batwing. The ordeal resulted in Cassandra being cleared from the suspicion of Slade and her father’s influence and being officially adopted as Bruce Wayne's daughter.
With Batman's invitation Cassandra began working in the Outsiders team and, despite some initial mistrust from some members due to her recent activities with the League of Assassins, she was accepted as part of the team. However when Batman suddenly disappeared without word or warning, Cassandra took it upon herself to lead but the team was fractured from an attack by the Thomas Wayne and a shunned Cassandra was left to return to a demolished Batcave.
With the absence, and now suspected death, of Batman, Cassandra created the The Network as a countermeasure to his absence. This action was not well received by Nightwing, who was having trouble coming to grips with his step-father’s possible death. The two would eventually sort out their conflict after Alfred intervened on a fight between the two. Cassandra handed over further creation of the Network to Nightwing.
When the third crisis hit, Cassandra was among the heroes responding to an Article X draft to combat the rise of Darkseid. While it is unknown what her role in the struggle against Darkseid was, it was during this crisis that Batman’s body was found in Darkseid’s chambers; the hero having shot the god with a poisoned bullet and then succumbing to Omega Beams. The cloak and cowl worn by Batman in his final moments was returned to Wayne Manor by Superman and Wonder Woman after the crisis was abated.
With the Batman gone, Gotham City soon became overcome with a re-energized crime wave that was encouraged by rumours that the Batman was dead. With the The Network fully activated the Bat Family and their allies keep the city together as the mantle inheritors (Richard Grayson, Jason Todd and Timothy Drake) work out who would become the new Batman; resulting in Grayson becoming Batman and Damian becoming the new Robin.
Unconvinced that Bruce was dead and alienated from the Bat Family now that Damian had taken his role as Robin, Timothy Drake becomes Red Robin and goes in pursuit of clues into Bruce’s circumstance. Meanwhile Cassandra pursues Bruce Wayne’s orders that she was instructed to follow in the event of his death; passing her costume and the Batgirl mantle on to Stephanie Brown, she then leaves Gotham via the Gotham Airport to an unknown location, eventually resurfacing in Hong Kong where she is operating as an unaffiliated vigilante although she still kept in contact with Timothy Drake and, following Bruce Wayne's return, re-received her Batgirl Uniform from Drake as he visited Hong Kong on a business trip.
- One is where she became Batwoman and pursues a partnership with then Batman Timothy Drake who himself harboured a wish to pursue a relationship with her.
- Another is shown in the future of the Titans of Tomorrow where she; as an alternate Batwoman, is part of a Titans Army, an army of 2nd/3rd generation heroes that patrol America with an iron fist. This future overrode another future where Cassandra had been killed by Duela Dent and the Titans Army did not exist.
- In an alternate time, where Thomas and Martha Wayne were not killed, Bruce Wayne never became Batman; and in extension Robin, Nightwing and Batgirl where never created. Since in this timeline Ra’s al Ghul became the emperor of the world Cassandra herself was most probably never born.
- In an alternate world Ra's al Ghul unleashes a plague upon the human race, killing the majority of Earth's human population. In this world Batman is killed by Ra's and the members of the Batsquad (Cassandra Cain, Jean Paul Valley, Timothy Drake, Selina Kyle, Richard Grayson) are similarly dead. Bruce Wayne and Talia al Ghul's son, Tallant, however, eventually rises, confronts and defeats Ra's and his plague.
- In an alternate version of Earth 12, members of the Bat family are part of an underground resistance movement against the totalitarian regime of Vandal Savage. Cassandra and Timothy Drake are both seen, along with Richard Grayson and Barbara Gordon, when the Earth 12 Justice League arrives at their underground headquarters with underground faction leader Bruce Wayne.