in a way you could call her the mc. but no, you can only change the formation. you really only have 4 characters. ryu, selena, lisa and nadia.
and the only 2 that really do much are selena and lisa
You get 2 more companions, but they're very close to the end game. Basically, every female character with a portrait becomes a party member at some point, so you also get the blue-haired policewoman, Laura and the pink-haired gangster, Maya, in that order. Honestly, If you got Nadia, Laura's not too far off, and likewise for Maya.
That said, unlike Ryunosuke, Selena, and Lisa, whom are considered the true MCs, Nadia, Laura, and Maya are more-or-less party fillers rather than love interests. They each only have their initial skill tree to work with, and they have very, very few lewd scenes.
Of the three, Laura is easily the most well developed in terms of the story, while the other two are very lacking, especially Nadia. However, I personally feel that Nadia is probably still the best combatant out of the three, especially considering that if you've been grinding up until they join, the new party members are still gonna join at much lower levels than the main party you've been running, and they don't gain experience if they aren't in the main party afterward. However, the game itself always treats them as if they're with you at all times, so you can still see all of their events from start to finish, regardless of party formation.
Unrelated, but I'm still waiting for an answer on this:
I'm doing the Perverted Cape quest in Monicabel, and I apparently need to find Julia. I know she's in pretty much every brothel, but those are just lewd scenes, and don't appear to be tied to any quests.
How should I progress?