Board Thread:Questions and Answers/@comment-27176987-20160913054504/@comment-25942629-20170205194645

Yu's "mom" is actually not his real mom. That's not revealed until book 7 of the Catastrophe light novels, but Yu isn't aware of it. We don't know much about his origins.

Seraph genes were passed down through the generations to awaken those individuals as seraphs when humans break a certain taboo. They essentially punish and destroy humans for the crimes they commit. (The anime changed things by having Yu fight Crowley while awakened, but in canon they only target humans and disregard vampires entirely.)  The sin that triggers them may be the revival of the dead, but things aren't entirely clear yet.

However, the greedy Thousand Nights and Demon Army sought to take control of these immensely powerful angels of destruction, and they call this project the Seraph of the End Project. In the manga, the Demon Army has managed to awaken two of them and control one.