Fish appears as a regular thin fish with black-colored eyes, alongside a shark-like fin at the top, alongside a massive tail fin.

On some occasions, this design will instead be replaced with a larger variant. This massive variant will also have the 'Fish' name replaced with a new name, that being 'BIG Fish'. Fish can also come in different colors, and can be randomly selected each time the NPC enters the elevator.  
Fish would enter the elevator after an opened circular light floods the elevator, and will then say it's entrance dialogue. Players can use the Spray Bottle and presumably the Petal Cone on the NPC as well. It will also sometimes jump in the air.

I see fish as just some regular chill dude with a laid-back voice, so try to do that!

https://regretevator.fandom.com/wiki/Fish#Current Wiki for fish!