SPOILER: HIGHLIGHT TO READ.

Archer is Emiya Shirou from the future. Shirou made a pact with "The World" (Gaia) in order to gain powers to protect some people. In return, he would have to serve Gaia as a Heroic Spirit. Because he isn't very well known, he became a certain type of Heroic Spirit called a Counter Guardian. As a Counter Guardian, his job was to be summoned to disaster areas throughout history as part of the clean-up crew, killing every human being in the area. In the end, he hated what he became.

Saber has always been a girl. History always had a tendency to make females into males. In other words, blame the chauvinist historians for not honoring women.

I have no idea. I'll probably ask Arai. Hopefully he won't kill me. I remember hearing about it, though. Might be some type of "special" type of seaweed.