Thief gets much better later on.
No Back Attacks, can see hidden passages and potholes (not necessary if using a guide, but still useful), can steal shit from enemies (especially Genji from Gilgamesh, assuming you get characters who can equip the shit).
Flee is great to just GTFO out of fights cause they are annoying
Otherwise just stick a sub-class on them (Summon is a good choice already) to help out.