Sounds a lot like Wizardry 4. But in both of those it's still a party against a bunch of bad (good) guys.
You see the rare female MC porn RPG play like that; usually it's a one-action-per-turn affair and they give you some party members one way or another. Giving a solo female MC a not-endgame way to take two actions is not uncommon in those games, but starting with more than one from the get-go is rare (but I think I've played one that did that, but I'm not certain). There are some fundamental balance problems with being one character playing against many in the traditional RPG milieu. Think of things like the standard status effects. It takes some effort to make instant death or petrification work when the PC only has one character. If you're already committed to not using those effects or doing the work of making them something they're traditionally not, that's one thing. As you mentioned, one of the reasons that you have separate characters is so that you can have separate roles for them, which brings in the turn order and character speed dynamics that would be missing if the party was just one character, among other things. Then you're only building/equipping one character. Without some kind of paradigm shift (each "action" gets its own equipment set and has its own place in the turn order, or something) you're limiting what is usually an important part of the traditional RPG gameplay. I guess the argument would be that this game's gameplay would be highly nontraditional, but that sort of game is the province of risk-taking devs. Also having just one PC in a party makes the traditional party interaction dynamics awkward. The Boss PC could certainly have noncombatant characters as foils, and they could be too weak to fight alongside him, I guess, or something like that. I think the type of game would certainly be doable (and probably has been done by some indie dev I know nothing about) but I think you'd end up with either a shallow rehashing of standard RPG memes that didn't work very well or some outre thing that would have to rewrite a lot of jRPG genre conventions to work or replace them with different gameplay entirely.