Rider

[Spoiler] Perseus [/Spoiler]

is the Rider-class Servant of Aro Isemi‎ in the Second Holy Grail War of Fate/Prototype.

Although he has sociable manners, is easily moved, and is a gentle hearted young man, he has the flaw that, once motivated to do something, he will not stop until he gives it his all and then some more. In this sense, he is also an anomaly. Once he decided to stain his hands with wrongdoing, he became capable of killing innocents with a gentle manner and smile.

(Source: Wikia)