You earn SOUL naturally from leveling up. This is used to gain new skills and shiz, but some skills are only obtained through side-training. Each character has their own personal sensei and there are some that are shared across the characters. It’s a good idea to do these, some of the skills you learn are very helpful. They are always depicted as a little human figure on your submap, so you can always find them.
[WORK IN PROGRESS]