Assassin is the servant of Major Reiter in the Imperial Capital Holy Grail Strange Story of Fate/KOHA-ACE, and Major Reiter in Fate/type Redline. He is also summoned by Ritsuka Fujimaru in the Grand Orders of Fate/Grand Order.

Overly boastful of his own abilities, he has a tendency to look down on all other swordsmen, but reputed as 'One with the sword, swift and nimble as a falcon,' his sword arm was the genuine work of a genius. He's extremely fond of sake and gambling, and has a rough and hasty personality. He looks down on people for his genius with the sword, and hates greatly being made light of.

He doesn't like using the head and there are occasions in which he depends on the decisions of others, and he tends to obey blindly fellows who make him enthusiastic once. Originally he’s unsophisticated with a chivalrous spirit and has a kind personality, but since he was used as others pleased and abandoned in the end he is twisted.

If you get on with him he surprisingly feels like a nice guy, but perhaps because he's violent and not smart, he’s the type that is used as people please.

He clearly doesn't like being mocked, or made fun of by others. In Fate/type Redline, when Tsukumo mocks him that any bum can become a Servant, he brutally beat her up for it. He finds sentimental and heart-warming moments disgusting that makes him want to puke. He tends to uses dirty tactics against Servants if it's necessary to throw them off guard.

(Source: Type-Moon Wiki)