Along with Barnacle Boy and Grandma SquarePants, Mermaid Man is one of the three supporting characters who have been retired from the series. His character was retired after his voice actor, Ernest Borgnine, passed away in 2012. He and Barnacle Boy now appear as toys or on pictures.
Mermaid Man is a parody of both Aquaman and Batman. He wears a nearly identical costume in every episode (two shells on each side) and his powers, which include being able to control the creatures of the sea, are also similar to those of Aquaman. Mermaid Man and Barnacle Boy's TV show and method of fighting crime are based on the campy 60's Batman TV show, their secret headquarters, the Mermalair, is a parody of the Batcave, and their vehicle, the invisible boatmobile is a parody of both the Batmobile and Wonder Woman's invisible jet.
Of note, he and Barnacle Boy are virtually completely human in appearance, except that they are the same size as the other characters of SpongeBob SquarePants, unlike the humans in the two movies.
Mermaid Man, as well as Barnacle Boy, are both humans, and it has been confirmed that they once lived on land. Mermaid Man has a circular body and thin legs. His costume is very similar to that of Aquaman, having an orange shirt, green pants, and seashells on his nipples. He also wears pink slippers due to his age. He wears a purple starfish on his nose and has a purple belt with a metallic blue "M" on it that has the ability to shrink or incinerate people, unlike Aquaman.
Mermaid Man and Barnacle Boy are in Rock Bottom when Patrick and Mermaid Man have a soda drinking burping contest. Patrick wins but Mermaid Man gets very bad gas that makes him unable to stand up and SpongeBob finds help to get him in the invisible boatmobile to leave Rock Bottom.
Mermaid Man and Barnacle Boy appear during a new episode being directed by Gill Hammerstein. In it, they are falsely accused of destroying the Sand Stadium when it was actually stolen by the Sneaky Hermit. However, when they are sent to Bikini Bottom Jail, the prison warden tells them what is really going on and lets them go so they can catch the villain, thus accidentally letting the Dirty Bubble and Man Ray go free.
Mermaid Man and Barnacle Boy go to Goo Lagoon and to the Krusty Krab to find clues. When they go to the Chum Bucket, they discover that Man Ray and the Dirty Bubble are using the mayor's child as their captive in an attempt to help the Sneaky Hermit's plans. They reluctantly make a truce with the villains and work together to find the Sneaky Hermit with some help from an expert hunter from Jellyfish Fields.
They manage to track down the hermit and foil his plan of putting Bikini Bottom on his back so he can have a home. When the crisis ends, they return to the Sand Stadium to continue watching the musical performance.
Mermaid Man is an old man who is overweight and very round in shape. He exhibits several typical characteristics of an elderly man, such as white hair, poor sight and hearing, crankiness, and memory loss, often forgetting what he is thinking about or doing. Elements of his costume not shared by Aquaman include an M-shaped belt buckle that is metallic blue and his belt is purple (several versions of Aquaman, however, wear an A-shaped belt buckle and his belt is yellow), a purple star-shaped mask, a scallop-shell bra (which is also a cell phone), and a pair of pink fuzzy slippers. The purple-shaped star over his nose was given to Mermaid Man by a mermaid. It is the only way that he can breathe underwater.
Mermaid Man has a sidekick, Barnacle Boy, whom he still treats like a child even though Barnacle Boy is 68 years old (according to "Mermaid Man and Barnacle Boy V"). Mermaid Man also notably goes crazy whenever anyone says "evil."
According to Mermaid Man, his powers all come from his costume. His abilities include summoning and controlling sea creatures, high-speed swimming (used to create the "Raging Whirlpool"), water balls, super strength, and the ability to survive underwater. While Mermaid Man retains all of these abilities, they are not as powerful as they were in his prime.
In addition to this, his belt features a wide array of abilities, similar to Batman's utility belt, as seen in "Mermaid Man and Barnacle Boy IV." These abilities are usually manifested as a beam or ray. The only one specifically named on the show is the "Small Ray," which shrinks things, but various effects are created when SpongeBob tests various buttons on Squidward in an attempt to reverse the Small Ray's effects. These include several bizarre mutations and effects, such as causing the victim to suddenly catch on fire, turn inside out, get a clump of snakes covering their head, or to be cut in half by scissors. Despite being the operator of the belt, he doesn't know how it works
It also contains the power of Wumbo. Wumbology is the study of everything Wumbo. Wumbo means the "opposite of mini," as seen in "Mermaid Man and Barnacle Boy IV."
Also, in the video game Lights, Camera, Pants!, Mermaid Man states that his belt has a "Chin Cam," a hidden camera that takes pictures of his chin.
In-universe merchandise based on him
In the episode "Ditchin'," a Mermaid Man and Barnacle Boy origin comic book is shown. One panel features a man resembling Barnacle Boy and a woman resembling Mermaid Man in a spaceship. The woman has two children in her arms resembling Mermaid Man and Barnacle Boy, implying that the heroes are brothers.
Mermaid Man and Barnacle Boy have several supervillain enemies. Their two most notable foes are Man Ray and Dirty Bubble. Other villains include the Atomic Flounder, who is now retired, the Sinister Slug, the Jumbo Shrimp, Kelp Thing, and the Moth, a humanoid moth who steals light, as seen in "Night Light." Mermaid Man claims to have been trying to catch the Moth for 57 years due to his very small size.
The invisible boatmobile is a parody of the invisible jet of Wonder Woman and the Batmobile of Batman.
His friends and enemies are similar to Batman and Aquaman's' friends and enemies such as Barnacle Boy, Elastic Waistband, Miss Appear, and Man Ray being parodies of Aqualad, Robin, Plastic Man, Elongated Man, Wonder Woman, and Black Manta.
The fight sequences have scenes with words like "POW!," "BAM!," and "NAP!," another possible nod to Adam West's portrayal of Batman.
His utility belt seems to contain gadgets for any situation, much like Batman's.
His sidekick, Barnacle Boy, is a parody of both Aqualad and Robin.
His supposedly fake origins (shown in "Ditchin") suggests he is from outer space.
He and Barnacle Boy spring into action after they "say the oath" while touching the tips of two golden rings. This is an obvious parody of The Wonder Twins both with the rings and the oath "Wonder Twin powers activate!" It should be noted that the Green Lantern Corps of DC also say an oath, and wield "power rings" as their armament.
The way he shouts to the place he wants to go is a parody of the famous way Batman shouts "to the Batcave," mostly in Adam West's TV show (sometimes, followed by the singular eye-catch).
He is very similar to the DC and Marvel superheroes, Aquaman and Namor (though mostly Aquaman).
In the episode "Mermaid Man Begins" it is revealed that Mermaid Man's real name is Ernie, a reference to his voice actor, the late Ernest Borgnine.
A common stereotype of the elderly, Mermaid Man is easily annoyed, falls asleep often, and is very senile.
On the DVD episode listings, his name is spelled as "Mermaidman."
When it comes to mermaids, seashells are used in place of bras or bikini tops (as seen with Ariel from Disney's The Little Mermaid). Mermaid Man sports a seashell bra, thus continuing his mermaid motif.
The running gag of the way Mermaid Man says, "Eeeevil" was ad libbed. When Ernest Borgnine was recording his dialogue, when he was supposed to say "Evil", his voice cracked, causing the engineer and voice director to go into a laughing fit.