Rin is a model student and idol of Shirō's school. Secretly, she is a powerful sorceress and Master in the Holy Grail War. Rin is the descendant of a long and old line of sorcerers and therefore lacks a formal magic education. However, she is extremely powerful in her own right.
In the anime, Fate/Stay Night, she ends up becoming Shiro's ally. She is able to use special gemstones that can be enchanted and explosive. It is also revealed in the beginning of the anime that she originally wanted to summon the most powerful servant, Saber, but that she instead ended up summoning Archer.