It looks like the anime won't be able to include her. She is obviously a future summon, a dragon with metamorphosis ability to turn into humanoid shape: type Horny Girl. I wonder if the protagonist just powerlevels later summons using cheezy tactics or exploits, or if he naturally encounter stronger and stronger summon candidates during his travels. I suppose Melfina is numero uno, unless she was "balanced" too hard by the devs- I mean author!