From what I have seen, you can select Reimu, Youmu or Kaguya at the beginning of the game as your main character.
You can only play as Kaguya after you beat the game. Extra Mode also gets unlocked when you beat the game.
Having played the game myself (as Reimu), I have to say it's a really terribly designed game. Now, the graphics and music (excluding sound effects, but that's just my opinion) are fine enough (I find Flandre's stage theme actually catchy), but the level design....ugh. Most times, enemies come rushing at you (notably the lance fairies, with you having barely any time to react. Kisume (Suwako's stage) literally comes without warning. The shots from various rabbit cannons come super-fast and often without warning, effectively knocking you into a bottomless pit. By the way, you can die from spikes in the previous or next screen. There's also the fact that
Beating the game unlocks Kaguya and an Extra Mode, which literally adds more enemies into a game with already shoddy level design.
Bottom line: This game just isn't enjoyable, mostly due to shoddy level design. Of course, this is just my opinion.
Here are the weaknesses (at least as Reimu):