Fish Costume Mask

This is a fishing quest reward. Hand a quest fish to the Angler and the prize roll can come back as the costume, arriving complete instead of one part per turn in.

That makes it a question of showing up. You get one quest per in game day, so there is no way to speed it along beyond never skipping a day. If you are already grinding him for the Golden Fishing Rod or the Angler Tackle Bag, this will land on its own eventually.

Vanity only, so none of it defends you and all of it sits in the social slots over your real gear.

The head part swallows your whole face and the skirt hides your legs, so the outfit changes your outline completely rather than recoloring you. It is a full silhouette swap, which is rarer than it sounds among the joke costumes.