At the level select screen, press RT to switch to Coco.
When in a level, kill enemies from above with the butt bounce (jump + B). After the fifth enemy, the cheev will pop. I got this easily in the Turtle Woods level, but I’m sure it can be done in most of them.
If the guide can be improved, please add a comment. Thanks in advance.
EDIT: As EaseAndInspire has noted "You have to be on the second world, ice world and go up to the pink platform to unlock her. Took me a good five minutes to figure this out "
Thanks for pointing that out!