Rin ftw <3
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 secretly a Magus and a Master in the Fifth Holy Grail War. In the beginning of the anime, she summons Archer for her Servant, although she originally wanted to summon Saber. Rin is reared as the successor to her family’s magecraft, instructed by her father Tokiomi Toosaka to prioritize sorcery over her own interests.
Two centuries ago, the Toosaka House, in collaboration with the estates of Makiri and Einzbern, helped found the Fuyuki Holy Grail Wars. The land of Fuyuki City, which possessed the second greatest spiritual power in all Japan, was owned by Rin’s ancestor, Nagato Toosaka; at the behest of the Einzberns, he provided his property as battleground for present and future wars. Nagato was a kirishitan, as well as a pupil of Zelretch; these titles gave him considerable influence in both the Mage’s Association and the Church, allowing the Toosaka family to participate in the Holy Grail Wars, free from harassment by either organization.
Rin was brought up as the successor to her family’s magecraft, and was instructed by her father to prioritize magecraft over her own interests. Ten years ago, Tokiomi Toosaka was selected as a Master in the 4th Holy Grail War, and Rin was entrusted to take care of the Toosaka estate until his return. Her mother, Aoi Toosaka, was left in a debilitated state from the war, so her care was left to her guardian, Kirei Kotomine. After her father was killed, Rin continued to improve her sorcery with some guidance from Kirei Kotomine. To divert attention from her private life, Rin has diligently strived to earn the admiration of her peers, even though she is secretly stingy, tomboyish, and a slacker.
The rights to the Toosaka lands were inherited by Aoi after Tokiomi’s death, and subsequently inherited by Rin after Aoi’s death. Kirei managed them as Rin’s guardian, and because of his foolhardy and coarse management, the majority of profitable properties ended up in the hands of others. It is possible that he believed the wealth was bad for her upbringing due to following the principles of humility and poverty.
Although Rin resents the sacrifices she has endured for the sake of tradition, she is fiercely competitive and eager to prove herself as a skilled and capable magus. She was immensely disappointed at summoning Archer and ended up in a quarrel with him; however, the duo soon reconciled, set aside their differences and quickly got along.
For the sake of her deceased father and her family’s name, Rin seeks to obtain the Holy Grail, though she has little personal desire for it. Unbeknownst to her sister Sakura, Rin feels guilty and believes that she has abandoned Sakura after she is adopted into the Matou household, so much the guilt was what driven her to revive Shirou, Sakura’s target of affection, at the brink of death.
Rin seems to exhibit this in regards to Sakura. For someone who is usually very intelligent and observant, she is remarkably poor at noticing all of the things that have changed about her sister, including even her hair color. When she goes into the Matou house for the first time, she immediately realizes what being the Matou heir means, and yet despite knowing that Shinji has no potential as a magus, and that her sister was adopted into that family because her father couldn’t train both of them, she fails to make the obvious logical jump to figuring out Rider’s true Master or her sister’s situation until it is explicitly pointed out to her. Of course, since working it out would have meant either hand-waving the fact that her sister is suffering horrific torture, or breaking from her duty as guardian of Fuyuki in an effort to save her, it’s perhaps understandable that she decided not to think too hard about it.