Ritsuka was a candidate for Master at the Security Organization for the Preservation of Humanity, Chaldea. As one of the low-level novices, he was summoned to Chaldea simply to fill their quota for potential Masters. However, the tragic explosion that devastated Chaldea left him as mankind's sole surviving Master and since then, as he was the only one left capable of Rayshifting, he has taken up the mission to eliminate the Singularities and save humanity's future.

(Source: anime.fate-go.us)