At the temple gates awaits the guardian who's sole purpose is to not let servants in or out of the temple. Assassin is summoned by Caster to be guardian of the temple, and bound to the mountain on which it's located. With no master to sustain him, Assassin's time is limited. In the past he was supposed to be Sasaki Kojiro, and Assassin doesn't bother to hide that fact. He also reveals that Sasaki Kojiro never existed; he was simply a foil for the legendary Miyamoto Musashi. Heroic spirits (and villains) are elevated to that status through the collective minds and beliefs of people, which may not be real.
Assassin is aware he has no past, thus has no desire for the grail as he never lived. He has a wistful thinking similar to what Sasaki Kojiro was thought to have, and merely wants to fulfill his desire to face a worthy opponent. Occasionally he let's people pass without fighting them due to ambigous orders on how to protect the temple, but also because lesser opponents don't interest him.
Sasaki Kojiro was noted for using a nodachi, a sword much like a katana, but larger: particularly in length. Nodachi were not very common as they required more strength to use, and were at a disadvantage at speed (being larger), but was a weapon Sasaki was famed to use. Assassin's technique is formidable because of it's lightning speed coupled with the longer range of the blade.
His noble phantasm is known as the "Turning Swallow Cut". Sasaki Kojiro was said to have perfected a sword technique that could cut a bird in mid air. It consists of three strikes that happen nearly simultaneously from different directions and can't be countered due to the fact that it comes from different sides.