In a world in which each person has a Holy Grail that can summon a Servant, Karin has summoned Kijyo Koyo. She is Erice Utsumi's close friend. Karin lives in Shibuya, where she attends middle school, 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. She has siblings.
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 Koyo openly and had to communicate through telepathy. She first met Koyo'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 Koyo is cute and is happy that she finally met Koyo 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.
Karin is fourteen years old. She wears her swimsuit under her normal clothes, since she spends so much time in the resort town Akihabara. She was taller than Erice until only recently.
When Karin was nine years old she used to have her hair cut short and wear boyish clothes, a Sukajan with short leather pants. She was also slightly tall for her age. When they first met back then, if Erice hadn't heard her voice she would have mistaken Karin for a boy.
Karin is a cheerful and understanding girl. Erice describes her as having the power to effortlessly lighten the mood. Patience is far from her greatest virtue, and often her emotions will flare which will lead to a fight with Erice, but neither girl ever lets it stay that way. Erice calls them irreplaceable to each other, and if one ever lost the other, that would be the end. She does not speak much to Erice about her family or any other friends she has. She cares a lot for Koyo's wellbeing and wants her to feel welcome, which is why she does not spend much time at home. Koyo describes Karin as a strong-minded and brave woman.
As one of the new type of humans born with a Holy Grail inside them, Karin is able to change her body's physical age with Command Spells and will never die of natural causes, though she can still be murdered. The new style of Command Spells that are used "After" the Holy Grail War regenerate over time.