The main character is "you," and there are a group of NPCs that you can team up with, all optional. I've never tried a solo run, but the game is supposedly do-able that way, too.

You can play as one of a half-dozen pre-made characters with unique skills/voices/backstories, or create your own custom character. Throughout the game, you get the opportunity to recruit almost all of the pre-made characters to your party, as well as a few more non-starter characters, and then you adventure with squad of up to four characters at a time.

The backstories and personalities on each character are enough to keep it interesting narratively, and the branching decision trees across the game vary wildly. The depth and variety of builds is pretty solid. My first playthrough took about 100 hours while still missing a few big things. Since then, my partner and I (because it does have up to 4-person miltiplayer) have played start-to-finish a couple more times. I think she's hovering around 2,000 hours all-in, because she's done a few solo playthroughs as well.