Initially, Sebastian is loyal to his brother, but he is easily persuaded by Antonio, his father's brother and the usurper of Prospero's dukedom, to plot against Alonso and take over the throne of Naples. They plan to kill Alonso while they are sleeping in a grove on the island. However, their plot is foiled by Ariel, Prospero's mischievous spirit.
Sebastian is portrayed as a weak and gullible character who is easily swayed by others. He lacks the strength and courage to stand up for what he believes in and is quick to follow the lead of others, especially when it serves his interests.
Throughout the play, Sebastian's character undergoes some development. After their plot fails, he becomes remorseful for his actions and expresses regret for participating in the conspiracy. He eventually reconciles with Prospero and Alonso and is restored to his rightful position as a prince of Naples.