For every other character, adding % increases the odds that they have a scene with Ryoka that day, and reducing it lowers the chance they are selected.Edit: also, what does using charms on the shrine grounds do? it says in the tips that it makes you more aware of what is going on but what exactly does that help with? is there any more content to be accessed by doing this?
You could pump one guys percentage to 100 and others to 0, if you want to go down that route specifically.
As for what they do for the MC, I'm not 100% certain, but it should act as a deterrent on events happening to Ryoka. Not sure how exactly it works, but it neither unlocks new content or affects the old content (like texts and stuff). So mostly you don't need to bother with it.