These are the characters you can unlock in the game.
These items can be won by getting more than the points listed in the minigame or by making the particular trick shot.
These are items hidden on the courses that you can unlock
These are the courses you can unlock in the game.
To gain easy loyalty with your characters, simply go to Challenge mode and find a VS match to play. Once you start, hit Start and choose the Give Up option. Do this three times and you will lose the match. There is no downside to this, but your loyalty with that character will go up.
Sometimes while playing as Mizuho, it will automatically switch to her twin sister (Mizuki). Mizuki has a completely different personality from Mizuho - it is sometimes interpreted as Mizuho having a split personality. That is not the case, she just has an alternate hidden character.