1. Mad Dog

Cain’s first experiment is his adopted son Mad Dog. Cain drove him crazy with his abuse and decided to discard him. One of the guards spared Mad Dog but set him into the wild and basically raised him as an outdoor pet until Nyssa al Ghul convinced Mad Dog to join her League. Cass and Mad Dog have a fight, Cass ends up knocking him out but gets stabbed and dies in the processes and Shiva has to stick her in a Lazarus Pit. What happens to Mad Dog after Cass defeats him is not shown.

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2. Annalea

Annalea is an assassin for hire also trained by Cain. She’s gets introduced by seducing and killing a man, before Cassandra beats her up and kills her. Apparently Cass is so jealous that Cain loved someone besides her she murders her sister and then tries to force Tim to kill Cain. Thankfully it’s later retconned that Cass turned evil because of brainwashing instead of jealousy, but Annalea’s still dead lol.

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3. Marque and the Cain/Deathstroke expiraments

During Batgirl Vol 2 we meet Cass’ older bio sister, Marque. Ever since escaping Cain, Marque has dedicated her life to hunting him down and killing him.

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Speaking of the evil!Cass arc, Cass meets her while she’s also hunting down Cain, as well as Deathstroke, to get revenge on them for brainwashing her into becoming a killer. Cass learns that Cain has been raising even more girls and also giving them Slade’s super strength serum, training them to take down the meta-hero community.

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Marque was hunting down these girls and murdering them, giving herself a scar for every “hint of Cain’s blood” she wiped out. Between Marque’s killings and Slade deciding to cut his losses and blow up their base, a majority of them died, but Cain does say there are still some left.

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Anyways, Cass stops Marque from killing Cain by beating her up in an alley and seriously injuring her, which makes Marque vow that Cassandra is her enemy, not her sister.

Good question! Initially during the Batgirl 2000 run Cass finds out that Cain tried his cruel experiments on other kids before he had Cass, which makes sense. Then in her painfully badly written evil arc she suddenly decided that him having other kids was a huge betrayal that she hated him for and she started killing them. They may have mentioned in that arc that some of the kids were younger but I'm not reading that shit again to check 😭 so I'm just going to assume Cass was the youngest because it makes the most sense, even if some of the resentment indicates that he might have been raising them alongside Cass. Even Cain's newest trainees in Batgirl 2008 don't look any older or younger than Cass. Marque looks... around the same age as Cass and says that Cain stole her older than the other girls. But Cass does act older than her in some ways.

So honestly she could be older or younger. I personally prefer her older, because Cass being the youngest makes the most sense to me, even though she's clearly more experienced and mature than Cain's other kids. The writing was patchy at best for the whole arc so I do think you can just go with whatever option you prefer.

Cass Cain Birthday week day one: Teen Team / Yellow