A few things, there are three rare item merchants, the last one one cedis's side of the destroyed bridge.
Exclusive weapons and armors can be upgraded but that's only when you've taken control of cedis, the third rare item merchant sells it's recipes.
All characters has two secret skills but most of them are worthless, except for a few. The only skill truly deserved to be considered secret skill is Rein's second one. It deals MASSIVE damage on a single target five times, sth truly worth the cost of 50SP to max level.