Nixie is a bug that you summon from a relic, which follows you around and performs attack animations without dealing any damage. The problem with it being a companion is that it has the wrong name and takes up an action slot. Let Nixie be the pacifist companion she always wanted to be, please.
I think I saw Mo doing at least some dmg, Croacker I am not sure, Nixie I never saw doing dmg so I don't use her.
Some explanation in tooltip could be helpful, to help us decide which is worth having in action slot.
She is not fighting, she is just playing around and love throwing water at other.
On serious note all of them seems to do the same amount of damage, for lvl 95 about 6k each hit on rare and 6.5k each hit on epic. All of them seems to be able to kill lvl 40 elder on their own and deal 50-60% to lvl 50 elder HP(~100k damage) for the whole summon duration. One thing I notice is that their attack really often miss for some reason, especially to higher level enemy.