Haku took part in the quest, got his/her share of experience, and evolved from that.
I literally didn't remember any plot from this series like haku who? you wouldn't kill anymore? huh? what? and unfortunately the way MC act is not motivating me to reread
Haku is a child of the dragon god. Who is kind of a pervert and lusts after the MC. We aren't sure who is Haku's biological mother, but MC adopted Haku.
About killing - the girl in question used to be an assassin, but then the guild caught her and rehabilitated her.
MC has S-rank case of pretending to be normal, but she's actually an overpowered protagonist.
Overall, I feel the story should be much funnier than it is. And I'm not sure whose fault is that.