Medusa (メドゥーサWP, Medūsa?), Class Name Rider (ライダー, Raidā?), is the Rider-class Servant of Sakura Matou in the Fifth Holy Grail War of Fate/stay night. Due to Sakura's unwillingness to participate as a Master, Shinji Matou is instead given control over Rider.

Concealing her eyes with a mask, she is a hard one to understand. However, it is not that she doesn't have much in the ways of emotion; rather, she is the type that thinks too much and ended up stuffed with emotions inside. In battle, she is ruthless and without mercy, willing to prey on unrelated innocents without hesitation if it's by her Master order in the Holy Grail War. On the other hand, she displays a degree of kindness to those who are not antagonistic towards her Master.

Rider is silent, sultry, blunt, and vigilant, never hesitating to shield her Master from harm. She is always faithful to her Master, even with Shinji acting in that role most of the time. She is not openly talkative, and she prefers to analyze people and their actions. She is very protective of Sakura and willing to sacrifice herself for Sakura's happiness. She is very logical and she doesn't act out of anger or instinct.