Detective Synth was created to replace Leo's psycho mom. Leo's genius psycho dad decided to rebuild his whole family. He wheels her out as a birthday present more or less, after stuffing 16.7% of the great plan into her head, and poor cinnamon bun Leo goes "MURDEROUS MOMMY MAKE HER GO AWAY GO AWAY". Sibling/Mia synths take care of him while dad takes her out to the woods, tells her to scamper, and then *kills* *himself* without even doing the courtesy of synthing himself first. (Not really healthy family dynamics.)
Beatrice -- the youngest but most advanced model of them all -- scarpers, alone, unloved, becomes Karen, the cynical, sharp, and knowing detective who only drinks to fit in and has the most beautiful apartment one could imagine. The others believe she was destroyed before Daddy E killed himself.
When she's reintroduced to the collective, they all say "but we would have loved you" and Leo would have gotten over his "freaked out to his core" reaction. Relations at the end of Series 1 remain cool between the collective and Karen, who prefers to stay with her favorite human and pick out aubergine headboards.
I love Niska, who has adopted a "black panthers" attitude about refusing to sell out and "act human", even to save her life. Unlike Hester, she's not a psychopath. She loves books and music and German girls, and probably wouldn't kill just to tidy up some loose ends...
Unlike (also) new Dr. Evil from America, who is happily disassembling and killing sentient synths so she can fulfill her sick twisted dream of resurrecting her daughter. Not sure if she's re-creating the daughter in Vee or she managed to do a partial Leo without the tech to re-implant, but I'm pretty sure Merlin is going to end up on her table and either be dissected or close to dissected so she can get the daughter she feels she's owed.
I don't get what Mia sees in Cafe Boy other than she's programmed to find people who need her and fix their lives. It doesn't seem like an equal or balanced relationship, and if they were human, I'd be telling him to run away.