Greetings, same problem here.
Missing characters/monsters: AureliaBlue Mimic Goo Slime(defeated but cant remember unlocked) MimicChair
At least I tried my best to unlock=w=
well actually, you should be able to unlock them after the domination scene :D
if you head to the forest of Lys, you may find them to fight, then after a complete domination scene will be unlocked!
Anyways, I will check it out to find some way to improve the gameplay and unlocking after the game is completed 😉