True Assassin

The real Heroic Spirit of assassinations.
He was summoned by Matou Zouken, and came into the world using Sasaki Kojirou's body.
With his white skull mask and black robes, he is the stereotypical assassin.
Ability-wise he's a little underwhelming, but he's an easy Servant to use. In contrast to his unsettling outward appearance, he's actually quite loyal, and will faithfully serve somebody he's acknowledged as his master to the bitter end.
Because he is merely one of many wraiths (Heroic Spirit candidates) swarming around the name “Assassin,” his basic parameters are low. To compensate for this, he modified his own body, and so was able to stand against the other Servants.
There is no face beneath his mask. His skin and nose have been shaven off, transforming him into “nobody.” That is what it means to become the Old Man of the Mountain.
When he was first summoned, he barely had any intelligence at all, but by acquiring Lancer's heart he rapidly powered up. Afterward, he was influenced by Lancer's personality.
By the way, it seems that he was particularly attached to his dirks, because he would always take care to gather them all up before returning from a battle.

Fate/side material: Fate Encyclopedia