In a world in which each person has a Holy Grail that can summon a Servant, Karin has summoned Berserker. She is Erice Utsumi's close friend. Karin lives in Shibuya, but she spends much of her time in Akihabara where Erice lives. Karin's parents rejected their Holy Grails and keep their Servants unsummoned, and so Karin spends little time at home because she wants her Servant Kijyo Koyo to feel welcomed, since her parents don't take well to such a bizarre-looking Servant. Erice and Karin first met when they were 9. She was kidnapped by Louis XVII and was trapped and drugged within his Noble Phantasm during her visit to Akihabara. Karin is also considered one of the few rare surviving victims of the month-long kidnap incident as well, but as she was in the midst of running away from home, their parents may not have filed a police report on her disappearance. Due to her strict family rules over not allowing a Servant to be summoned, she was forbidden from communicating with Kouyou openly and had to communicate through telepathy. She first met Kouyou's manifested form for the first time during the kidnap incident, shortly before she was released. Her physical form did not scare her even though it's also not what she expected. Despite that, she thinks Kouyou is cute and is happy that she finally met Kouyou for the very first time. Louis' death released her and saw Louis dying under Erice's arms. This was the first time they met and have been friends ever since.