Tohsaka Rin

Rin is a model student and idol of Shirou's school. She barely talks to other students in her school and exhibits a desire to be left alone, as exemplified by her tendency to stay on the school's rooftop, away from the rest of the students.

She is also secretly a magus and a Master in the Holy Grail War. Rin is reared as the successor to her family's magecraft, instructed by her father, Toosaka Tokiomi, to prioritize sorcery over her own interests. After her father is killed, Rin continues to perfect her sorcery, with some guidance from Kotomine Kirei.

She strives to earn the admiration of her peers to avoid drawing attention to her private life, even though she is secretly stingy, tomboyish, and a slacker. She is sharp, resourceful, and avidly competitive.

From Wikipedia

She also appears in Fate/Extra CCC as a hidden, optional boss. This is not to be confused with the Toosaka Rin in the same game.