These are all of the Guest characters you can encounter in this game. To unlock them, you must beat them in their respective sport after earning a medal in the Tournament Mode. Note that these characters aren't playable in any other sports.
To unlock the ability to choose your music for events, you must beat all Level 1 tournaments.
Not all of the game modes and content are available from the start. To unlock certain modes, you need to fulfill certain criteria. Princess Daisy gives you a hint about this when she talks about the sand sculpture that Chao and Toad were building. Tournament Mode is Mii only at first, but Mario and Sonic veterans will gradually start to join in at Level 2, and can eventually be unlocked for use at Level 3.