The Flying Dutchman is a ghost who has a green glow around him, yellow eyes with black irises, a scraggly beard, a pirate hat with a chunk taken out; a handbag marked "Souls," a droopy nose, a coat, little pigtails, and a ghostly tail, and also speaks with a West Country pirate accent. He has a sister named Sally Dutchman.
During all of his appearances, the Flying Dutchman proves to be anything but a "friendly ghost." Though his role in the series is to be the devil of the sea, constantly varies from a sea demon to a restless soul or even as a psychopompic judge to the dead, the Flying Dutchman has been consistently shown to be gruff, malicious and even downright sadistic, scaring people and stealing their souls out simply for fun, and in his first full appearance, he attempted to steal the souls of everyone in the Krusty Krab simply because SpongeBob's ridiculous costume as the "Flying Dutchman" was the ultimate insult to his image. The latter trait shows that he's extremely prideful.
Despite his antisocial behavior and malevolence, the Flying Dutchman isn't pure evil or without companionship.
He has a number of ghost friends and has the capability to bond with other people in case they have the same interests or objectives (mostly SpongeBob). He is also a notorious womanizer, with not even mortal women being free from his teasers (though the former episode implies that he's divorced). While the Flying Dutchman takes strong delight in scaring people, he doesn't live exclusively to haunt mortals, having sometimes tormented them only out of boredom (scaring SpongeBob for months to wait for his ship to be fully repaired), or otherwise because someone provoked him. Although, he is no saint, the Flying Dutchman has a strong sense of justice and has been known to award people for good deeds done towards him  or to punish wicked souls (taking his role as a Psychopomp very seriously), and has been known to keep promises as seen in "Shanghaied," when he grants three wishes to SpongeBob and the gang in exchange for his sock.
Powers and abilities
The Flying Dutchman is shown to be one of the most powerful supernatural beings in the ocean, rivaling King Neptune. Though The Flying Dutchman's skills vary from episode to episode, he has been consistently capable to levitate, and teleport in a cloud of smoke preceded by a storm. In his first appearance, he is shown to be capable of stealing souls and fire-breathing through his nose. He has also shown to be capable of opening dimensional portals to banish his enemies. He has shown to be capable of manipulating matter (even turning SpongeBob and Patrick into ghosts and making objects appear out of thin air) and granting wishes.
However, of all his incarnations, his most "frightening" appearance was in "Ghost Host," where he was able to shape shift into numerous horrific forms and warped reality effortlessly. The only limitation to his powers regards his ghost tail. Despite being a champion in looping for centuries, he has long forgotten how to tie his shoes and it's later shown that he can't eat without his "dining sock."
But the most powerful thing he can do is seen in Ghoul Fools, he takes out one of his eyes and transforms into an atomic bomb, the nuclear power of this attack is so powerful that the explosion will send the victims to the void, but this can also kill him despite him being a ghost, because he isn't seen in any other episode after this. This means that it is a one-time occurrence.
The Flying Dutchman makes appearances in nearly all SpongeBob video games. His first appearance was in the very first SpongeBob video game, Legend of the Lost Spatula as the final boss. He also appears as a boss in SuperSponge and as an enemy in SpongeBob SquarePants: Operation Krabby Patty. He appears in The Yellow Avenger, he is in Rock Bottom and gives SpongeBob a mission to find his 3 keys for him.
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He is named after the legendary ghost ship, The Flying Dutchman, and seems to resemble the famous pirate Blackbeard (Edward Teach) or the ship's captain and mythological character Davey Jones, although according to the legend of the Flying Dutchman the ship's captain is a man by the name of Willem van Der Decken.
He doesn't like it when his beard is shaved off because he fears he would get pimples again.
It's also said his beard grows in 1,000 years, but it grew back in several months. His claim could be an exaggeration, though.
In "The Curse of Bikini Bottom," he looks through his closet for a shirt and finds one that is sentimental because he says "It was the shirt I was buried in." However, in "Squidward the Unfriendly Ghost," his origin, from the comic book, says he was never buried and his body was used as a window display so technically, it is impossible for the shirt to be the one he was buried in.
However, he may have indeed been buried eventually, and he remains as a ghost due to having been one for so long.
The Flying Dutchman says he hasn't worn shoes for 5,000 years, so which means that he could be over 5,000 years old. That's older than King Neptune, because he is 5,000 years old.
The Flying Dutchman has to have his Dining Sock on to eat.
In "Shanghaied," it is shown he has a monocle to read
The Flying Dutchman appears to sometimes get emotional, though this very rarely happens.