Trooper souls are bought whole (one time only) or as fragments from the trooper store. Get them each and every time you can.
Much like Eternity souls, Trooper souls give you a guaranteed 5* toon from a select list we like to call the "troopers"
It's important to note you get 100 Fronier coins each time you open one of these.
All four trooper toons are needed to collect US Cavalry Eddie from the Trooper dungeon, leading up to Major Malakhov.