How do the characters affect the music?
music is not affected, but they do have an effect on how well you do on a stage...
Agi charas lets you get further on FMS (field music stage), as you chain more notes, they get to walk faster, depending on their AGI, usually best to have him/her as your party leader, and slowest one last.
Str charas do well on BMS (battle music stage), every good+ note you get, they get to hit the enemy, and tend to have more hp, so it lets you take more hits.
Magic charas, they can do support or offensive magic during BMS, they are dependent on chaining notes to cure, or nuke the mobs, but are somewhat squishy and their str ratio isnt as good
Luck... IDK, how often you find treasure?
there is also another stage mode, VMS (video music stage), you get a video and follow some notes on it, some abilities work here, but otherwise I find these dull and borring :/