Koujaku was born in Midorima living with his mother and from that point his life is insignificant until he meets Aoba when Aoba is like six or seven and being made fun of for looking like a girl and that they don’t believe he has nerves in his hair (he can’t help looking like a girl he is a twink with nerves in his hair) and Koujaku makes them go away. So they were friends for a long time after that point but then Koujaku was forced to move to the mainland because of his father.
Koujaku's father is a yakuza boss and his mother was only his mistress but his wife couldn’t have kids so Koujaku is the only heir. Back on the mainland Koujaku didn’t want to take over his position so to bind him to it his father made him get a tattoo from a guy called Ryuuhou. And Ryuuhou is obsessed with tattooing, it's his life's passion . He doesn't care about anything else or morals or ethics or anything. He just loves tattoos and is evil. So either way he’s getting a tattoo on his back from Ryuuhou.
But Ryuuhou previously had been hired by Toue to test out mind controlling people through tattoos and Ryuuhou was like heck yeah I can make my art living and breathing which is pretty epic and based. So Ryuuhou put’s a mind altering tattoo on Koujaku that makes it very difficult for him to control his anger and Ryuuhou lets Koujaku know that and it makes him angry he tries to kill Ryuuhou and accidentally ends up killing his mother. He was really guilty after that so he tried to kill himself but then he was like nvm Aoba needs me probably so he goes back to Midorima and is then working as a hairstylist pretending he likes women and also is obsessed with protecting Aoba because all the best people love this awful man (awful man is Aoba) and being internalized homophobia. He also runs the gang Beni-Shugure now and they are all super loyal and stuff. In his good route he gets to overcome his homophobia though so he can get freaky with Aoba because duh.