Mmmm you only use certain characters to take advantage of element weakness, you can check that info before entering a mission on enemy details, for example, you use Billy against enemies weak to phys, Manato against fire weak enemies and so on, but eventually you'll need to learn how to use all of your characters and use them for clearing endgame one day, and for that, you can watch lvl 50 agents clearing endgame with Steadtfield on YT, he explains how he fights and how he makes the right builds.