He is the most well-known "unknown" mathematician...:)

I think I first read about him in a Martin Gardner book as a kid. So yeah, not that unknown.

Kind of same here -- I think I first read about him on the Argentinian magazine "Humor y Juegos" (Humor and Games) some time in the late 70s/ early 80s. I guess most people with any interest in recreational mathematics/ puzzles will know of Ramanujan.