When you say you're replacing the character, what exactly are you doing? Is it just a prefab swap in the combatant settings or are you messing with the actual prefab data? Or are you deleting things from the project and reimporting?
Check your animation types/set up in ORK against the actual animation controller on your character. @GiL can correct me if I'm wrong (since I don't have it in front of me right now), but I think the brown and blue pants guys (players, enemies, and npcs) are all using the same controller and animation settings. If you delete the animations or controller, it'll be deleted across all of them - which could be happening if you're deleting the brown pants guy to add your custom guy.
Or it could be in the animation settings in ORK. If you change the animation settings in ORK to update them to the new guy (like rename idle to IdleMan to fit the custom controller setup), and if the same animation set were being used by the enemies, they won't work unless you either also change the enemy's animation states to match or make them their own animation set in ORK with the correct names.
I'm not sure I explained that well at all. :c