Heroic Spirit of assassinations. Unlike the other classes, the Assassin class is only ever supposed to be one particular Heroic Spirit.
The Assassin in the story was a samurai guarding Ryuudou Temple's gate.
Sasaki Kojirou was summoned into this class solely because his summoner violated the rules.
Strictly speaking, he is a wraith, not a Heroic Spirit. It seems that in the past, he was a master of martial arts with some connection to Ryuudou Temple.
His true name is unknown. Or rather, it would be more accurate to say he didn't have something like a true name in the first place.
Perhaps because he never had the possibility of serving a lord, he selflessly devoted himself to the sword, and eventually attained a technique with the same level of mystery as a Servant's Noble Phantasm.
In terms of pure swordsmanship, he was the strongest Servant in the Fifth Holy Grail War.
Because he was summoned using Ryuudou Temple itself as a catalyst, he cannot leave the temple gates.
…Hmm? Doesn't this feel a bit like a haunting?

Fate/side material: Fate Encyclopedia