Okay, since there have been a few topics about this I'll try to explain how these work.
If you look at a unit, you'll notice that they might have an empty slot for a potential (a dash, a strike-through indicates they don't get a 4th potential), even though they're at level 20. This is a hidden potential that can be unlocked by accumulating 5 points. The 5 points are made as followed:
- Participating in Battle (1 point)
- Killing an enemy unit, Healing an ally, Repairing Cover (ie. a sandbag) or Tank (2 points)
- Capturing a base (3 points)
- Saving a fallen comrade (4 points)
For an easy way to unlock their hidden potentials, go to skirmish 1. Use one unit, and kill another allied unit. Then collect their body to "save" them. After this is done, select another unit and kill the unit that you first used, and subsequently saved their body. Repeat this until you've done it with every unit that you want.
After you've acquired enough points for a hidden potential to be unlocked, go to Castlefront Street and talk to Ellet. She'll mention that she interviewed a rising star from your squad, this is how you know you got the hidden potential for your units.
Rinse and repeat.
I took the point values for the hidden potentials from "zldafnatic"'s post on a previously closed topic, so cred's to him.