Kokatsu has not directly killed anyone, however she has helped Okibo set up actual patrol routes and in doing so helped him kill people, while at first she did have issues with this they were small as she knew that Okibo only killed the worst of the worst and eventually adopted a similar mindset to his.
(Just to be safe, I do not endorse nor encourage vigilantism, terrorism or arson)
(Look! A character with a stable home life! It's like a unicorn!)
Age:0-15
Honestly not much happens compared to the others, Kokatsu's parents were both very successful as they both owned and operated a fairly large business and while they did their best to be good parents their job made It difficult because they were rarely ever home, as a result Kokatsu developed a strong independent streak, whenever she had a problem she would rather deal with it in her own way than get help from an adult.
For example, when she was twelve a new kid transferred into her school and started bullying the other kids, rather than letting her parents or a teacher deal with it she instead recruited 17 other kids and had them make the new kids life a living hell until he transferred out.
This ended up being the point where she found her true calling in life, plotting and scheming. Other than a couple of times where she mobilized her class against other students not much ever happened, after all she killed time by reading and watching anime, not moonlighting as a vigilante(Okibo) or a terrorist(Kyojin)
Age: 15-17
At this point she had developed a reputation amongst the students and teachers alike, "if you mess with Kokatsu then you mess with the entire school" but eventually, inevitably, there will always be someone that's either brave enough or dumb enough to step out of line.
In this case it was the P.E. teacher who decided it was a good idea to try and grope her, (it wasn't) this was probably the only time she had ever bothered to take an issue to an adult, and the only reason that she did this time was because her classmates were cowards that would call the cops on her if she asked them to help burn his house down, and so she tried the legal route, sadly "there were no cameras or witnesses so it's your word against his."
(And the fact the the P.E. teacher was the principle's cousin didn't help)
And so Kokatsu found herself outside of a well known foster home after hearing about a new kid that moonlighted as a vigilante, after entering the house it took her all of 2 seconds to spot him (she'd have to be blind not to) and from there it only escalated.
After helping Kokatsu with the teacher (he didn't die but he went to the hospital and moved out of town after he got out) Okibo and Kyojin, sensing a kindred spirit, quickly latched onto Kokatsu, eventually after trying (and failing) to escape from their friendship or drive them off she gave up and decided that she may as well help Okibo with his patrol route, and it would be for the best if she was to try and get Kyojin to target places and organizations that deserve it instead of random places that couldn't survive his chaos (Translation: they grew on her but she refuses to admit that she thinks of them as friends)
Over the next couple of years (After she realized that she had severely underestimated Kyojin) Kokatsu spent most of her time either trying to keep Kyojin from burning the country to the ground or playing games and watching anime to de-stress so she doesn't have a heart attack from trying to keep Kyojin in check
(The fun thing is, Kyojin respects her, if she asked him to stop he would)
Age:18
Kokatsu hangs out with Okibo and Kyojin for her birthday... And then she sells her soul to a God.
Personality:
Mastermind: given the choice Kokatsu prefers minions and lackeys over friends and as a result she always keeps others at arms length, or at least she used to before those idiots mucked it up, nowadays while she still tries to avoid getting attached to people she has been far less successful, though in high stress situations she does still tend to revert back to being cold and calculating.
Responsible:
Somehow, in spite of her best efforts Kokatsu has ended up as the mom friend, at this point that is the only role she knows how to fill in a friendship, she's the one that makes sure that Kyojin isn't injured when he gets into some ridiculous situation that he has no right to survive, and the one that helps Okibo when he gets stabbed, and the one that beats the crap out of anyone that harasses her friends ("They're not my friends Kyojin! They're lackeys, allies at best, the only reason I'm helping them is so they'll owe me!!")
Nerd:
She is extremely smart and spends a significant amount of time reading, watching anime and playing video games. ("Call me a nerd again and I'll kick your a*s Okibo, I don't care that you're bigger than me!")
(To summarize: Kokatsu used to be extremely cold and calculating, she made a point of not getting attached to anyone and saw people as pawns, then Okibo and Kyojin happened, and suddenly she had two people that didn't care if she didn't want to hang out or eat lunch with them, were too intimidating for her to turn her class against and who refused to leave her alone, eventually she ended up getting attached to them and despite her best efforts began to think of them as friends, and ot was downhill from there.)
(Behold!! A tsundere!)