Will never truly have a defined look, but I’m fond of thinking of them as a simple mint green flab similar to a headless flab in colors.

I imagine they change species and costumes but retain their mint green coloration to some extent, and regularly change back into a flab when they so feel like it.

Another identifying feature would be wearing their dark blue bathrobe on top of whatever other costume they have on. It’s not worn for temperature regulation so much as a comfort object that provides Fifteen with a sense of security.