I ended up with Kay/Griffith, Esther, Signy/Aliss/Diego, and Mort most of the time. Signy, Aliss and Diego needs no explaining; they're there because they deal damage.
Kay when I want my team to be empowered 24/7, and with the occasional shield, Griffith otherwise if I want a tank.
Esther is just superb once you unlock her skill morph that lets her deal 250% base damage on one target (2k!). You're basically guaranteed a kill every two turns, and while you're only limited to cardinal directions, you get unlimited range instead. Also comes with a deployable tank that can distract in a pinch, and some other more situational abilities. Handy, all in all.
Mort has superb mobility, and he comes out of the box with skills that let you adapt to any situation accordingly. An attack that silences(!). Huge AOE attack that gives him shield. A Pure attack that ramps up to ridiculous levels with debuffs. The only underwhelming thing is his ultimate, which simply copies a debuff onto every enemy on the field. Very situational, but you can copy things like Stunned or Silenced.
Grenn and Mifune on the other hand, are so one-note that taking them is often a liability. Dwarf has shit movement range, and while being able to stack attacks is cool, his damage is so shit that I ended up using him as a pseudo-tank instead. Grenn is... Grenn. No idea how to use him. Vanish takes one turn. Stabbing people takes another. Damage underwhelming even then. Stun on demand is okay, I suppose, but then you find Vanish has a long cooldown, which kills his usefulness completely anyway.
Will try the other diplomacy characters next, Henrrieta seems to be very strong.