The female lead charcacter is the the previous demon king who also got betrayed. She got her body cut down in five part, and each part was sealed inside a labyrinth (and yeah, it didn't kill her). When she meet the hero again, she's just a flying head, trying to recover her body.
Well the MC is weakened as well, as he can't muster his power of hero anymore. The two of them decide to team up and to take revenge againts the ones who betrayed them.
What I like about her, is her personality, especially her inhuman sides: She's fying head and
she's hinding it by making subsitute of a body with magic. And I love the way how she likes make the MC freak out when she dispel her magic body, leaving only her flying head.
It's very refreshing to see a non-human female character who does'nt act like a normal human.... And not like a random japanese, highschool girl from manga/anime!!! [god know how much I hate it!]