Sebastian is an animated anthropomorphic crab in Disney's The Little Mermaid film franchise. He is a red crab from the Caribbean island of Jamaica and a servant of King Triton. Apart from his role in the film trilogy, Sebastian features in the animated series, in the Kingdom Hearts series of video games, and even has music albums based on him.
When the film opens, Sebastian is announced as the "royal court composer", his full name being "Horatio Thelonius Ignatius Crustaceus Sebastian", and he conducts a concert that King Triton's seven daughters are supposed to perform. Ariel fails to show up for the concert, and later Sebastian joins King Triton in scolding her, blaming her for his humiliation. Sebastian is tasked by King Triton to follow Ariel and keep her out of trouble, and so witnesses Ariel saving Eric from drowning, which horrifies him as contact between the human world and merworld is forbidden. Sebastian accidentally lets this information slip to King Triton, who confronts Ariel and destroys her collection of human things in a rage. Feeling guilty for having betrayed Ariel's confidence, Sebastian follows Ariel when she visits Ursula the sea witch for help, and later helps her quest to win Prince Eric's affections. After Ursula disguises herself as the human Vanessa and wins over Prince Eric, Sebastian goes down to inform King Triton, and later becomes instrumental in helping Ariel and Prince Eric defeat Ursula.