Y/n is Hayley and Elijah’s daughter they didn’t know they had. Vampires originally can’t procreate, but some wutches did a spell of one night intimacy turning into, well Y/n. The witches thought that she would kill the Mikaelsons when she was old enough (y/n is three right now) haley did not give birth to Y/n as the transferred the child to some other witch.