This Kaito primarily comes from the song "Warum," but because he needed backstory and several of my friends are terrible people, they topped me into an idea I had been toying with: that his creator was Dr. Len Kagamine, from the Kokoro/Kiseki series of songs. Part one of that explanation is
here- I'll be putting Frankenkaito's adventures of trying to live among humans and details on how he was put together here once I've got the time to do so. ETA: OKAY I AM NOT EVEN GONNA BOTHER DOING ANYTHING FANCY WITH THIS, it is INFODUMP TIME. I don't guarantee good grammar either because I am totally out of it.
Shortly after escaping from Dr. Kagamine's lab, he ran across a young girl drowning in a river. He pulled her out and attempted a very clumsy recucitation, which killed her. Her father found them, and seeing what appeared to be some sort of monster attacking his child promptly shot Kaito, who then ripped him apart out of pain and anger. (He took one of the girl's hair ribbons and the bullet he was shot with as mementos.)
Perhaps a week or so after this, he managed to run into Gakupo, the older man who taught him how to speak and read. (At this point Kaito could understand language, but not speak it due to his various coordination issues). In addition, Gakupo attempted to teach Kaito how to behave honorably as well (although a good percentage of those morals didn't really click for him until later). They lived together for about a year, and then Kaito was careless about showing himself to the neighbors, who then notified the authorities. The local police came; Kaito misinterpreted their intentions toward Gakupo with more hostility than was actually there, and he attacked them. Gakupo tried to stop him, and in the scuffle he and the two policemen were killed. Kaito fled, only coming back a month later to burn down the house of the neighbors, who he blamed.