Sup, Meow. Thanks for your comment.
The game really leans into the idea that the girls have lives of their own apart from the MC. That doesn’t mean you can’t get romantically involved with them or even end up together in the future. PDA is very much about choices, about making promises or letting go. The focus of romance here is on the development of love, not necessarily on its final outcome.
As for party members, the fixed ones are Mersia, Alyas, Cleo, and Boris.
The romance options are Mersia, Cleo, Melissa, and Kayla. But each girl has her own role, not all of them are adventurers. And if you pay close attention, only adventurers are allowed to enter the dungeon. Since neither Kayla nor Melissa are adventurers, they can’t join the party. And because the main focus of the game is dungeon exploration, it wouldn’t make much sense to include them as fixed party members.
I hope that clears things up.
Have fun~~