Oh, for AP characters, there's no skill-unlocking you have to do. Everything's there for you when you get them; you just have to distribute the SP accordingly.
However, it's still very possible to screw up an AP character's ST, as with the two Dio examples I've mentioned before. Another example: Zero's more lenient in this regard, but that would mean leaving out some of his passives for his Sword Mode, which allows for more consecutive hits with Grandark. He may have enough SP for everything if you don't even touch Classic Force, but that would mean missing out on HP Loss (a stab move with his sword that gains HP with a hit; useful for PvP).
Pinkie Pool's archnemesis: Oatmeal.