"Stephanie would say "Suck it up and deal!" Okay, Steph. For you."- Cassandra Cain, Batgirl #67
Oracle: "I worry you'll get hurt...or worse. I mean, poor Steph..."
Batgirl: "Don't. Talk. About. Her."
Cassandra: Why does everyone I care about die?
Stephanie: Because...you care about everybody. And everybody dies.
Relationship: Best friend, partner
Name: Cassandra Cain
Aliases: Batgirl II, The-One-Who-Is-All, Kasumi, Black bat
Height: 5'5 Weight: 110 lbs
Parents: Mother - Sandra Wu-San (alias "Lady Shiva), Father - David Cain, Adopted Father - Bruce Wayne/Batman I
Siblings: Annalea (deceased), "The Mad Dog"
Adopted Sibling: Dick Grayson (Robin I/Nightwing I/Batman III), Jason Todd (Robin II/Red Hood II/Nightwing II/Red Robin I), Tim Drake-Wayne (Robin III/Red Robin II)
Cassandra Cain, the second Batgirl, has amazing body-reading abilities that allow her to predict her opponents moves. This, along with her amazing martial arts prowess, is a result of being rigourously trained since birth to be an assassin by her father, David Cain, who did not teach her how to read or talk, substituting movement as a language. Cassandra ran away from her father after she was first made to kill, and upon meeting Batman decided to use her skills to fight crime. She later learned to speak thanks to a psychic, though she is still inept socially and in other areas.
Though initially stand-offish when they first met, Cassandra Cain soon developed a very close friendship with Stephanie, even training her without Batman's permission. Though there were many differences between the two (Stephanie had to work hard at martial arts while Cassandra's skill was effortless thanks to her assassin training, Stephanie was peppy and friendly while Cassandra was withdrawn and shy, Stephanie told a lot of jokes while Cassandra took things very seriously) they bonded over both of them being determined, good hearted and having massive daddy issues (Cass's father is an assassin, and Steph's is a criminal).
Cass was very grief-stricken by Stephanie's 'death', and has seen Stephanie twice in near death experiences.
Stephanie and Cassandra were reunited off panel, but Cassandra appears to bear no ill will toward Stephanie for her faked death and the two resumed their friendship and partnership. Shortly after the death of Bruce Wayne, the two friends were fighting a bunch of thugs together on the Gotham docks. The two worked together as seamless team. Stephanie teased Cass about her chattiness and Cass teased Stephanie right back about her inability to give up being Spoiler
After the fight, however, Cass became much more dour. Clearly lost and grief-stricken after Bruce's death, she took off her costume, saying she didn't want to fight for him anymore. As Steph looked away sadly, Cassandra suddenly told Stephanie the fight was hers now and vanished, leaving her costume with Stephanie and apparently handing over the Batgirl mantle to her friend. Stephanie decided to honor that wish and take up the Batgirl identity.
A one panel flashback shows that Stephanie attempted to follow Cassandra and persuade her to stay in Gotham, but Cassandra said "don't follow me, Steph. For your own good." before boarding a plane to Europe.
Tim Drake visited Cassanda in Hong Kong later on, where she was operating on her own as an independent vigilante. When he offered her the Batgirl mantle back, Cass declined, saying that Steph was Batgirl now and she needed it.
Summary of Cassandra and Stephanie's interactions:
First meeting: Batman introduces Steph to Cassandra while she is training in the Batcave. Stephanie is impressed by Cassandra's skill and asks if she has any advice, to which Cass merely replies: "Become faster." (Robin (88))
First team up: Cassandra recruits Stephanie to read a ransom note she found for her, and Stephanie insists on going along to bust the kidnapping, which turns into a gang fight/attempted murder. The two girls easily dispatch it, and Stephanie asks Cass to teach her "that trick with the Batarang" and offers to teach Cass how to read (this offer is never followed up on). (Batgirl #20)
The two team up again to take on Jokerized villainesses. (Joker: Last Laugh (3))
The two girl's friendship is cemented when Stephanie revives the "Shadow Thief" using CPR after a tearful Cassandra has accidentally killed him and Stephanie promises not to tell Oracle. (Batgirl #21)
Stephanie takes on a Lady Shiva worshiper who wants to kill Cassandra while Batgirl is out of commission. She imagines Batgirl following her around, egging her on, proving how much Stephanie admires Cass. She wins the fight after imagining being criticized by Cassandra, and then goes to visit the actual sleeping Cass.( Batgirl #26)
The two girls play "rooftop tag" and Cass repeatedly nerve strikes Steph (much to her annoyance) to keep her out of trouble or to stop her from interfering with Cass's investigation of the Bruce Wayne case. This causes her to realize that a nerve strike was used on the victim, Ms. Fairchild. To celebrate, Cass and Steph play MORE rooftop tag. (Batgirl #27)
Steph begins to train with Cass and they bond over father issues and become even better friends. Unfortunately, Steph has to end the training, which makes Cass sad. (Batgirl # 28)
Cass trains Steph without Batman's permission, and they have fun jumping rooftops and they talk about boys and their family, sharing secrets like Steph's father's death and Cass's dad being her legitimate father, but Steph gets involved in a fight when she thinks Cass is injured, and nearly gets knifed. Cass intervenes, catching the knife and knocking Steph out to transport her to safety. When Steph wakes up, Cass admits she thinks Steph should stop crimefighting as well. Steph goes away angrily, saying friends don't turn their backs on each other. (Batgirl #38)
They team up as Batgirl and Robin, best friends again. (Batgirl #53)
Steph comes to see Cass to tell her she was fired as Robin and thanks Cass for her friendship (Batgirl #54)
Cassandra sees Steph one last time before her "death" during the gang war. She tells Steph to go home and be safe, and Steph agrees, but Cassandra realizes she is lying and goes after her, only to be forcibly interrupted by further mayhem. During the rest of the gang war, Cass desperately searches for her friend, nagging Oracle and showing people her picture, and is devastated to find out about her apparent demise. (Batgirl (55))
Cassandra flashes back to Stephanie's funeral, which she cried at and reflects on how much she misses her.(Batgirl #58)
When the Penguin mocks Tim, asking him what happened to that "hot little blonde number" who was running around as Robin, and if she died too, Cassandra interrupts him by headbutting him. Later, she casually mentions to Tim that she and Stephanie used to laugh together about their fathers. (Batgirl #59)
When Batgirl nearly drowns, a ghostly version of Steph appears and convinces Cass to save herself. Cass is melancholy afterward, missing her friend. (Batgirl #62)
Cassandra thinks that Stephanie would want her to "deal" and ask Oracle for help. When Oracle mentions Stephanie, Cass reacts angrily as quoted above. (Batgirl #67)
After Batgirl is stabbed by "The Mad Dog" and dies, she meets up with the ghostly Stephanie once more, who tells her the secrets of her pasts and shows her what becomes of Bludhaven. Cass is pulled away from Stephanie when Lady Shiva dumps Cassandra's body in the Lazarus Pit and she is resurrected. (Batgirl #72- 73)
Steph and Cassandra appear to have briefly been reunited, though this is WTFInvisible!Steph and they show no sign of knowing each other, so until proven otherwise it IS NOT CANON. (Batman and the Outsiders (13))
Cassandra fights thugs with Stephanie and then, grief-stricken after Bruce's death, hands over the Batgirl mantle to her friend. (Batgirl (third series) (1))
More about Cassandra: Here