Neptunia is a female anthropomorphic fish from the Darkwing Duck series.
In "Just Us Justice Ducks", Neptunia responded to disturbances in the waters around St. Canard, and ends up running into Darkwing and his allies. Along with fellow reformed villains Morgana Macawber and Stegmutt, hero of Duckburg Gizmoduck, and Darkwing himself, she was invited to join a team proposed by Gosalyn: the Justice Ducks. Neptunia felt obliged to point out that she and Stegmutt were not Ducks, with Stegmutt commenting that he was one previously. At any rate, Darkwing himself rejected the idea of a teamup, only to learn his lesson the hard way when he attempted to defeat the villainous group known as the Fearsome Five.
Going off on her own, Neptunia found a section of the city flooded by the Liquidator, one of the Fearsome Five, and attempted to defeat him and liberate it. Defeated and captured, she was brought to Negaduck, who held her and her other would be teammates captive in machines designed to eliminate them. Fortunately, the humbled Darkwing came to their aid, saving Neptunia from a device that would have dehydrated her to the point of death. By joining forces and tag-teaming the villains, she and the other Justice Ducks were thus able to defeat the Fearsome Five.
Neptunia's origin is recounted in the episode "Something Fishy", Neptunia was a normal fish until she encountered a mess of radioactive waste as it was being dumped into the ocean. Exposure to the material resulted in her gaining a humanoid form, the ability to speak English, and the capability to breathe on land so long as her body moisture remains above a certain level. Traumatized by the experience and filled with wrath against the surface dwellers, Neptunia organized the denizens of the deep into a fighting force in order to strike back against her enemies. Fortunately, Darkwing Duck, Launchpad McQuack, and Gosalyn Mallard were able to turn her from her vengeful course.
Neptunia's concern for the oceans and command over fish are presumably a nod to the DC superhero Aquaman.
Of all the Justice Ducks members, Neptunia had the least appearances in Darkwing Duck, appearing only in her debut episode and the two-parter featuring the team she was briefly a part of. Stegmutt-or an alternate version of him-also appeared in the episode "Darkwing Doubloon", while Morgana appeared in several other episodes as Darkwing's love interest. Gizmoduck had been introduced in the DuckTales series, and appeared in a number of Darkwing Duck episodes. Neptunia's fewer appearances are fitting given her antagonism with Liquidator, who outside of his debut episode always appeared alongside the rest of the Fearsome Five.