Ohhh. Me and my big mouth!
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 the one to keep her mind off the human world. He is also the Court Composer.
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 often 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.
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, 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.
In Kingdom Hearts, after Sora, Donald, and Goofy first arrive in Atlantica, they see Ariel and Flounder swimming towards them, with Sebastian bringing up the rear. Sora tells Ariel that they're new to the area, so she volunteers Sebastian's services as a swim coach. After a quick complaint, he agrees to show the group how to swim. After the trio completes their exercise, a group of Heartless appear. Ariel swims safely out of the area, while Sebastian and Flounder hide in a nearby oyster shell. Once Sora, Donald, and Goofy defeat the Heartless, Sora decides to escort Ariel back to King Triton's Palace.
They arrive being chased by Heartless, which are promptly destroyed by King Triton. After Ariel receives a scolding from her father, Sebastian introduces Sora, Donald, and Goofy to his majesty. When they are questioned as to what brought them there, Goofy tells him they're looking for the Keyhole, which Ariel then asks about. As Goofy starts to explain, King Triton interrupts and tells them there's nothing like that in Atlantica. When Ariel questions her father, he orders her not to leave the palace. Ariel swims off, upset, with Sora, Donald, Goofy, and Flounder following after her. Once they all leave, Sebastian expresses his interest in the Keyhole to King Triton, who makes it clear that it's none his concern. Sebastian then reports to King Triton that the Heartless seem to be coming from Ursula's Grotto.
After King Triton argues with Ariel in the Tranquil Grotto, she once again swims out, upset with her father. Later, Ursula tricks Ariel into giving her her father's Trident, in exchange for passage to other worlds. She then uses it to attack King Triton. Sora and the group arrive, but are too late to stop Ursula. Sora decides to retrieve the stolen Trident himself, when Ariel says she wants to come along, since it's her fault her father was hurt. Sebastian also wishes to come along and assist.
Upon reaching Ursula's Grotto, Sora and company are attacked by a giant Ursula. Together they defeat Ursula and finally get the Trident back. Ariel takes the Trident to the Tranquil Grotto, revealing the Keyhole, which Sora then seals. Ariel tells Sora that, someday, she wants to go to other worlds too. Sebastian quickly says that, if she does, to leave him out of it.
In Kingdom Hearts: Chain of Memories, Sebastian has a much smaller role. While looking for Ariel and the Trident, he runs into Sora. He asks if he has seen either since they have both gone missing. Sora has not, but he decides to help search. Sebastian mentions asking Flounder, but he seems to have disappeared as well. He is later seen searching for them around Ursula's Grotto.
In Kingdom Hearts II, Sebastian's musical abilities from the movie are finally brought to light in the game. He is attempting to get Ariel's mind off the surface by way of music practice. Sebastian asks Sora, Donald, and Goofy to participate in the musical, saying she might settle down in practice if they're there. Sora and the gang then join in and participate in the singing exercises as well.
Ariel's mind is still on the surface, and she no longer wishes to practice. Flounder has the idea to cheer her up by moving the statue of Prince Eric that had sank into the ocean during a storm into her grotto. Sebastian is, of course, not okay with the idea at all, and says he'll have to tell King Triton. When he attempts to do so later, however, he becomes afraid and is unable to say anything.
Sebastian then writes a song to try and show Ariel how great the ocean is. The song, however, just makes her yearn for Sebastian to write a song about land and sea living in harmony. Sebastian gets frantic over what would happen if King Triton were to ever hear her sing a song like that. Ariel states that if he won't listen, then she won't sing at all, and she swims away leaving everyone behind. Sebastian then precedes to go off on a monologue, where he spills the beans about Ariel being in love with a human. King Triton then swims up behind him, just in time to overhear the big secret. This causes him to become extremely angry.
King Triton, in an attempt to get through to his daughter, destroys the statue of Prince Eric. Ariel, devastated, swims off, and soon afterwards meets up with Ursula. The sea witch says she can turn Ariel in a human if she signs a contract and gives up her voice. The contract states that Ariel has to kiss the Prince by sunset on the third day after she becomes human or she'll turn back into a mermaid, and she'll belong to Ursula. Sebastian attempts to talk Ariel out of it, but is unsuccessful. Ariel signs the contract and is transformed into a human.
Sebastian, having no other choice, tries to help Ariel kiss Eric in time. They are almost successful, until Ursula, who also transformed herself into a human, appears and uses Ariel's voice to trick Eric into thinking she's the one who saved him. Sora realizes this and breaks the shell with Ariel's voice in it, which allows Ariel to get her voice back. Eric realizes it was Ariel who saved him, but he does so too late. The sun sets, and Ursula, back in her regular form, drags the now-mermaid Ariel back into the sea. Eric then jumps in after them desperate to rescue Ariel.
King Triton shows up, and having no other options, gives Ursula his Trident in exchange for Ariel's freedom. Sora and the others then battle Ursula again. After Ariel and Eric defeat Ursula, King Triton sees how much his daughter is in love with Prince Eric, he allows them to be together. Seeing this, Sebastian complies with what Ariel had asked of him earlier, and writes a brand new song to show how the two worlds (the sea and land) can join together in harmony. During the ending credits, he attends Ariel and Eric's wedding.
In the original film, he acts as Triton's right arm and Ariel's advisor about the dangers of contact between merpeople and humans. Despite that, he is the true originator of Ariel's love with Eric, and helps her with this love, together with Flounder. In The Little Mermaid II: Return to the Sea, Sebastian has a much less important role, working only as Ariel's friend and fighting with the others against Morgana and her minions.