Neptunia is a fish superheroine. She is part of the Justice Ducks, though she has twice expressed displeasure with the name as "I'm not a duck." She mostly joined because it coincided with her goal of protecting the ocean. She was once a normal fish, but a run-in with some toxic chemicals mutated her into what she is today. She was nearly fried to a crisp when being held hostage by Negaduck in the episode "Just Us Justice Ducks."
She is often seen with an octopus companion named Hal. She shows not only the ability to speak English, but she can breathe out of water as long as her scales are moist, she swims at amazing speeds, and occasionally performs feats that require superhuman strength. She appears to be very independent and strong-willed.
Neptunia is the self-appointed protector of the sea. She is very irritable and has a short temper. She has an octopus companion named Hal, and uses a shell as a horn to command the creatures of the ocean to do her bidding. Originally an enemy to Darkwing Duck, she had a change of heart after she was proven wrong about him polluting her home and joined up with the Justice Ducks, though she was reluctant to help out at first.
- Neptunia was named after the feminine from of the word Neptune; the Roman god of the oceans and seas (the Roman equivalent of the Greek god Poseidon) and the 8th planet in the Solar System. She also possesses a trident of her own similar to that of Neptune. Even her role as the ruler and protector of the sea is based on Neptune and probably Aquaman from DC Comics and Namor from Marvel Comics.
- Her octopus friend, Hal, bears a striking resemblance to Billy the Squid from the Chip 'n Dale Rescue Rangers episode "Piratsy Under the Seas".