Real Name: Gatou Monji.
Gatou is a self-proclaimed scholar of all religions, and claims to have far surpassed all-known theology; he is a super truth-seeking monk.
He is a simpleton with a dynamic and unyielding personality. Unable to understand him or to be understood by him, he’s like a runaway train. He claims to have reached the height of knowledge in all religions and mythologies both past and present, but his core philosophies of seem to lie within Buddhism.
In this age of chaos and consumerism he earnestly pursued achieving the merciful heart of a Buddha, and the results lay before you; no matter how you look at it he’s a suspicious phony priest.
After undergoing the “severe training” required to become a hippie, he had a spiritual encounter in Arizona which inspired him to challenge the peaks of the Himalayas alone. Upon reaching the peak of a mountain, he met “the Primeval Woman.”
However, sadly, despite having a chance encounter with this fundamental divine being, there are apparently no human words by which her existence could be described or defined.
In order to introduce her as the Goddess of a new religion, Gatou made a request to Moon Cell, despite him not being too fond of it.
You may be surprised to hear that many years ago this guy was invited by Moon Cell as a highly talented and promising individual. WHY?

There is no use arguing the fact that his completely incomprehensible behavior tend to make people think of him as a savage that might attack them at any time. While Gatou loves fighting more than three meals a day; however he has absolutely no desire to kill anyone.
The reason he moves and acts like a Berserker in the Holy Grail War is simply that Berserker is the materialization of his philosophy “Enemies are to be defeated. Battles are to be won.”
If you were to meet Gatou Monji outside of the context of the Holy Grail War, you would just think of him as an annoying, but understanding and good natured guy.

Gatou is also a Master that shows up in CCC.
He promptly joins the student council and tries to fix the situation, just as expected of a super truth-seeking monk. He is quick to grasp every opportunity, and his only worry is about El Shaddai (God Almighty).
Leo makes him the pep rally leader, and in his free time after school you can find him behind the school building practicing cheers with several groups. Regardless of the situation in CCC, Gatou doesn’t lose sight of himself, and does whatever he can to the best of his ability.
He is unable to carry out his mission (…of spreading the religion of the primeval woman…), so he is far more relaxed than usual. Ironically, this situation that deviated from the main topic has brought out his strongest points.
Even though on the near side all Masters aside from oneself are enemies, on the far side of the Moon there is no reason to kill each other. The one that naturally accepted this matter-of-the-fact way of thinking, is Gatou.
This is digression, but during the days of his training he met Kiara in the mountains of Japan and had a zen dialogue match.
Kiara, a ditzy older sister type who is more or less invincible, was scared off by Gatou thinking “What the heck!? This guy is too much. Communication with him is impossible.” Gatou is the only one to have accomplished the feat of scaring her off. Just like the protagonist, Gatou has lost his memories of this incident with Kiara.
…Of course it goes without saying that the loss of memories and the amnesia is due to the meddling of a certain someone in CCC.

Fate/EXTRA Material: Encyclopedia of Fate/EXTRA