What if Tokoyami and Dark Shadow were supposed to be twins? And Dark Shadow's body was so weak that Tokoyami ended up absorbing her(DS is canonically a girl)??
Think about it; Tokoyami's quirk was supposed to be a bird mutation, while Dark Shadow's is somewhat of a shadow bird transformation/mutation. And they fused like how normal twins would inside the womb, but DS remained 'alive' because quirks.