You were a hybrid. A combination of human and vampire. Your mother was a human and your father was a vampire. Your mother has died after child birth. Your father was the only one left and he taught you how to survive.
One day, you went into the forest to drink some animal blood and you bumped into a wolf. Without second thought, you drink the wolf's blood and killed it in process. As you were drinking, you realised the wolf started to shift into a human and this confused you. Another wolf appeared and it called it's pack and it hurt you on your arm. The wolves were shocked to see your arm was healing so they knocked you out and sent you to Erwin, Levi and Hanji for report. Erwin wouldn't let you go until he was sure you were not a threat. He gave Hanji permission to test on you and Levi was to make sure Hanji didn't go overboard. You stayed in their pack and started to make friends..and maybe falls in love?