As far as I know, considering I'm in Chapter 3, he doesn't have a really... A big "Target" like he's not really the main. More or likely, it's Allegretto who's the "main" but seeing everyone is a protagonist....As for Chopin reason being for him is because highly doubted he ACTUALLY dreamt this in real life.
Chopin never takes the spotlight in the game. Well once, but it's not enough to call him a main. Honestly the game doesn't focus enough on anyone's back story enough (or at all too much really) for anyone to be considered a main.
If you really NEED the title of main handed to someone, it'd be Allegretto or Polka. More likely Allegretto seeing as you play him on the field more then Polka. But the story isn't focused on anyone...