After having been slaughtered by your girlfriend, who was secretly a fallen angel, you could say the love of your life or master; Rias Gremory revives you and now you're a demon, woo-hoo. Yada-yada-yada, you join the Occult Research Club.

You've now befriended Koneko, Rias, Akeno, Asia, Xenovia, Irina, Kiba, etc. I'm too tired to type all this down.