Tohsaka Rin

Current head of the distinguished Tohsaka family, and one of the main heroines of Fate/stay night. As a Master, she spent just as much time quarreling with Shirou as she did cooperating with him.
Her manner of speech is often hostile and dismissive of other people, but deep down she has a caring, big-sisterly disposition.
Her tendency of making trivial mistakes at the worst possible moment is a hereditary curse.
Though she really is a lot nicer than she seems, in the end she always makes her decisions based on cold, rational logic. In her own words, it's “not a very cute personality.”
There are even times when she thinks she might lack any sort of “girlishness” at all, and it's starting to become something of a complex for her.
In fact, her choice in personal clothes is probably an overreaction to that. Still, showing that much leg… a pure and wholesome boy won't know where to look. Please, try to bear with it, Shirou.
Her hobbies are admiring jewels and bullying Shirou. She's a mischievous girl at heart, so she loves teasing serious people. Please, try to bear with it, Shirou.
As far as magecraft goes, she specializes in the flow and conversion of magical energy.
Since it's not a magecraft trait well-suited for combat, she stores away magical energy in jewels ahead of time, and uses them as disposable, limited-function mystic codes.
Aside from that, she also practices pseudo-karate as a form of self-defense.
Her instructor in martial arts is Kotomine. That fake priest was responsible for the sequence of moves she used to corner Caster.
By the way, she had the most screen time out of every character in Fate/stay night.

Fate/side material: Fate Encyclopedia
§

Tohsaka Tokiomi's eldest daughter. The younger form of one of the heroines from the original story.
During Fate/Zero, Rin's level of understanding of the “Holy Grail War” is about the same as Shirou's was during Fate/stay night. That is, while it might seem like she knows what's going on, she really doesn't.
In the young Rin's eyes, her father Tokiomi was a great magus deserving of love and respect. She did not yet understand the ruthlessness and cruelty that stemmed from that ideal. If her father had survived long enough for her to comprehend that aspect of him, it surely would have had a grave effect on her own personality. But whether she chose to rebel against that inhumanity and turn her back on sorcery itself, or chose to embrace it fully, becoming a perfect, cold-hearted magus like her father—the only certainty is that for her, happiness would be nothing more than the fleeting dream of a far off parallel world.

Fate/Zero material: Fate/Zero Encylopedia
§
tohsaka-rin