I have a monster suggestion for NK power rangers.
Name: Isabelle
Gender:female
Weapon: none
Power: shape shift
Background: better stay hidden, the only thing I can say is that she was once a human.
She like infiltrate the NK since she can disguise herself as a human. Her natural look is that she's made of slime, a slime girl.
I thought that maybe as she tries to infiltrate the NKs she also starts to befriend them, then she turns on Monstrox and actually becomes their friend. When she becomes good she turns back to a human.