But the edge previewer complained about the lack of female party memebers...
In the build I played for preview I experienced 2 guest character join-ups (both Cor) and both lasted for less than 10 minutes, so... there's a big bloody difference between this and an actual party member, female or otherwise. Guest characters don't have skill trees or any of that stuff either. They have a few team-up attacks, but those are context sensitive. They're NPCs in the truest sense, where you can at least exert some control over your party members.
If we want to get into semantics: they're not party members, they're guest NPCs.