Dopplegangers

Doppelgangers
Dopplegangers, as described in chapter 27 of Blood Lad, is the other person in the world that looks exactly like you. One lives in the demon world, while the other in the human. Unlike the common belief that if you meet this person, you will die, you instead fuse into one body. In addition to this, the new body contains memories of both people. The main example of this is Fuyumi and Beru's mother Nene, who is both girls' mother fused into one body.

(Source: Blood Lad Chapter 27)