I don't have all the characters, only Merisiel, Kyra, Seoni, Amiri, Lini, Seelah, and Sajan. (Haven't used Sajan yet) I'll give my brief review of the characters I have.
(Disclaimer: I use the alt version of Seoni, Amiri, Seelah, and Kyra)
I find Seoni to be the strongest out of all the ones I've used. I use her for every mission, especially legendary. With legendary, if I'm just trying to do it quickly for daily challenges, I run with either Seoni + Merisiel or Seoni + Amiri. Spells automatically recharging is insanely strong, letting her blast away, and augury lets her scout pretty quickly.
Amiri is a beast and can basically win any normal combat on her own and extra movement is amazing on legendary.
Merisiel is useful for those levels where you need dex or stealth to avoid monsters or to close locations. She explores pretty slowly though. Too many items not enough allies or blessings.
Lini seems more like a support character to me, her 1d4+whatever makes her ideal for spamming buffs and recharging them. She's quite weak on her own and requires help from others to win combat. I gave her augury as well and it makes her an extra good scout with recharging animal allies exploring as well.
Kyra is situational, but can handle herself better than Lini. A bit slow on the scouting so I gave her an augury to speed things up. Very effective vs undead, but I never see the need to use healing. It's a waste of action economy.
Seelah burns through locations pretty quickly. Her damage could be better, and is an overall strong character both on her own and to the party. Just dont expect to acquire any boons from her location. She'll have it closed before you manage to get anything.
I would rate the characters I have in this order from best to worst:
2. Amiri / Seelah
Now all I gotta do is save up more gold to unlock all the male characters. I probably shouldn't have spent all my gold on alts.