I couldn't find this in the FAQs, so maybe someone can help.

I'm sitting on 100 souls, and not sure what to do with them.

I know technically how you can spend them for character chances, but I'm at a point in the game where a 3-star character is useless for me because of how hard the evolution pieces are to find.

Should I just sit on them and wait for a game update for a better use for them, or is there a chance for anything higher than 3 stars when redeeming them? I've hit a wall unless I can get another 4-star ally.

I'm guessing you can't combine them for a rare soul?