In Fate/Zero, Sakura is Tohsaka Rin's sister and the younger daughter of Tokiomi and Aoi. She was given away to the Matou family because the laws of the mage world prevented Tokiomi from training more than one of his daughters to carry on his family's tradition, and Tokiomi didn't want Sakura's potential to go to waste. However, Zouken is not really training her as a mage but instead putting her through hell. Matou Kariya made a deal with Zouken to win the Holy Grail in exchange for Sakura's freedom to return to the Tohsaka family, which he fails to do at the end of the series.
In Fate/Stay Night, Sakura is Shirou's timid underclassman and Shinji's adopted sister. She has been friends with Shirou since the previous summer when he was injured in an accident and she began visiting him with the excuse of taking care of him while he couldn't care for himself. Despite coming from the Matou family, Shinji claims she has been kept in the dark regarding magic as in a traditional magus household only the successor will be taught any magecraft (even though it's later revealed it's a lie and she's Medusa Rider's true master). Though she is respectful of her brother and defends his misconduct, he is in turn an abusive jerk towards her. Further, despite her upbeat attitude with Shirou, she is actually nearly silent and much gloomier when away from the Emiya household or Taiga and has no real friends apart from those two.
In Heaven's Feel, Sakura becomes the love interest of the story and secrets about her life and the Matou family are slowly revealed.
