Kariya is the white sheep of the Matou family and the first in many generations to have any real talent for magecraft. However, due to his disgust with his family he severed ties with them ten years prior, earning him the ire of family head Zouken Matou. Kariya has loved Aoi Tohsaka since they were childhood friends, so upon seeing Tokiomi Tohsaka apparently abandoning one of the girls he views almost as his own daughters he vows to protect her. While Kariya has no interest in the Grail, he made a deal with Zouken that should he win the Grail for him, Sakura will be taken from the horrible abuse Zouken intends for her and released back to the Tohsaka family.
However, despite his noble intentions, Kariya simply lacks the training to participate as an equal and is inflicted with parasitic worms that will boost his abilities in exchange for rapidly degrading his health. Making matters worse, Zouken forces him to summon a Berserker class Servant, all previously summoned examples of which have killed their Masters either directly or through the staggering power they consume in battle. Berserker and Kariya cannot interact directly due to Berserker's madness, but Kariya can issue orders, and sometimes glimpses Berserker's thoughts while sleeping, though these only disturb him.
