Traits

Your Perfpal's legs can be replaced with a mermaid's tail. The tail must be a realistic length. 
The tail must be that of a fish, not another aquatic life form.
(Aquatic, anamniotic, gill-bearing vertebrate animal with swimming fins and a hard skull, but lacking limbs with digits)

(This allows for an alternate "form" if you'd like for your Perfpal to have access to legs still.)