The Soul Badge is your reward for completing Koga's Genin Exam at the Fuchsia Dojo. You can also attempt Chuunin and Jounin training for rarer items after you clear Genin training, but you can also try Genin training again to improve your time.
If you try to put it on any character, it will cost 5 Skill Points, since none of the Trio have an affinity or a weakness to Shadow in poison form
The Soul Badge is unusual because it also has Light moves in psychic form as well as Shadow moves in poison form. It is mainly a defensive Badge, but also has some offensive and support moves.
Since all three characters are neutral to Shadow as poison, either one could make use of the moves on this Badge to flummox foes, slowly drain large foes of life, and lower their stats before they can even attack, making this an important badge for all three to master.
Its moveset looks like this (so far):
Level 1: Poison Gas
Level 4: Thick Smoke
Level 5: Poison Jab (5% mastered)
Level 8: Purify
Level 10: Toxic