So do you have a particular style of music you want for Dylan?
So, I checked out 'The Binding of Isaac' and to me the music is more modern/contemporary than, say, 'The Legend of Zelda: A Link to the Past' (to me, the original archetype for this sort of game). If you're unfamiliar with that music, check it out and let me know which you'd prefer.
Do you mean that you'd like to have a consistent main theme melody that can be played in different moods depending on the place/situation/Dylan, or an actual specific synth sound/instrument to use for all the lead parts - or both :P
Also, I'm completely new to this itch.io and game jams in general. Is this where we should be conducting these types of discussions or is there like a specific room/thread we should be using now that we've decided to work together on your game?