Ratharian, the Demon General guy does not look impressive on paper. He's a fighter with low attack, who doesn't do anything special on the first turn. But after running him a bit in a Demon build, he's actually super fun. His summon has to be one of the strongest in the game. He's fast, and has ways around Taunt.
Natalie Seline has been good for me too. I don't have any particular synergies. But she can blast hard and fast, and is the best healer in the game. And heals are surprisingly good at the rank I'm at, where it takes two turns to focus down a Merc.
Anyone else having some surprise successes?