Sebastian is a hermit crab with a Jamaican accent, who is in service of King Triton in Atlantica. He appears in Kingdom Hearts, Kingdom Hearts: Chain of Memories and Kingdom Hearts II. He is normally the one who tries to keep Ariel out of trouble, and keep her mind off the human world. He is also the Court Composer.
 Kingdom Hearts
When Sora arrives in Atlantica for the first time, he sees Sebastian swimming with Ariel and Flounder away from Heartless, and he teaches Sora how to swim, using Flounder as a target. When they arrive at Triton's Palace, Triton destroys the Heartless chasing them. Sebastian introduces Sora, Donald, and Goofy to King Triton. When Goofy tells Ariel that they are looking for the Keyhole, King Triton instantly interrupts and tells them there is no such thing and orders Ariel not to look for it. After Sora and party leave, Sebastian expresses his interest in the Keyhole to King Triton only to be met with a stare. After Ursula attacks King Triton with his Trident (which Ariel gives to her in exchange for transportation to another world), Sebastian and Sora arrive, but are too late to stop Ursula. Sebastian tells Sora and Ariel he wants to help them take her down. He helps them get into Ursula's Lair by pressing a button that was covered by a shipwreck that only he could reach. In the end, after Sora seals the Keyhole, Ariel tells Sora that she knows that someday she will get to other worlds, and Sebastian tells her to leave him out of it.
 Kingdom Hearts: Chain of Memories
Sebastian appears in the fake Atlantica in Castle Oblivion, albeit with a smaller role. He is looking for Ariel and the Trident as well since they both mysteriously gone missing. He later searches for them around Ursula's Lair.
 Kingdom Hearts II
Sebastian is trying to use musical practice to get Ariel's mind off the surface world. When Sora and the others side with Ariel, Sebastian gets mad, not wanting them to help her find Prince Eric. Sebastian then writes Under the Sea song in order to persuade Ariel to stay in the ocean where she belongs. Ariel is instead saddened by the song and yearns for Sebastian to write a song about land and sea living in harmony, making Sebastian frantic over what would happen if King Triton were to find out. Sebastian goes a little bit too far when in his monologue and spills the fact that Ariel is in love with a human. King Triton happens to be right behind him, and is deeply angered by the confession.
After Ariel changes into a human, Sebastian tries to help her kiss Prince Eric in time. After Ariel and Prince Eric defeat Ursula, King Triton finally allows Ariel to marry Prince Eric. Seeing this, Sebastian complies with what Ariel once said earlier in the sea, and writes A New Day is Dawning to show how the two worlds can join together in harmony. Afterwards, Sebastian attends Ariel and Eric's wedding.
Sebastian is a small crab, despite the numerous dissimilarities between him and real crabs. His design seems to be a conglomeration of several different animals, though he does vaguely resemble a crab. His claws somewhat resemble those of a lobster, the structure of his carapace is similar to that of a turtle, and he demonstrates in The Little Mermaid that he can retract his head into it, and, most notably, he has a head and neck separate from his carapace, which goes against the body structure of all crabs, whose head and abdomen are fused together.
Sebastian's salmon-colored, noseless face is vaguely humanoid, with a large mouth, large eyes, and a brow line, though he obviously does not have any hair. He even has a uvula and a pink, human tongue, other physical impossibilities for crabs. His claws and six legs are bright red, the top side of his carapace is maroon, and the underside is a dark orange color.
Sebastian is very strict when it comes to following rules, especially when the rules are given by King Triton himself. He is also very loyal (or maybe afraid) to his King and will do anything for him, even when asked to stalk Ariel, as he reports to him periodically on the ocean's state and Ariel's behavior. He also cares deeply about Ariel, often reminding her how bad the human world is and will try everything to stop her from getting there. In the original movie, Sebastian followed Ariel to the human world to try to save her. He can also be seen as Ariel's "conscience". He is very passionate about music and is often trying to lift up the undersea creatures' mood by singing or conducting an orchestra. He also has a tendency to let things slip out of his mouth.
In the original film, King Triton ordered Sebastian to stalk Ariel to prevent her from returning to the surface. However, she soon saves Prince Eric from a hurricane and falls in love with him, much to Sebastian's horror. Like in Kingdom Hearts II, he tells King Triton about this unwittingly, greatly angering him to the point where he confronts Ariel in her grotto and destroys everything she had collected.
When Ursula transformed Ariel into a human, Sebastian followed her to keep an eye on her, nearly being cooked by Chief Louis during this time. In the end, Sebastian attends Ariel's wedding, where he has another conflict with Louis.
In the sequel, Sebastian would spend the next twelve years with Ariel to keep Melody, her daughter, away from the ocean. His rivalry with Louis remains unchanged.