Antonio and Sebastian are two of the ship’s passengers who wash ashore on the island where Prospero, the magician, and his daughter, Miranda, live. Antonio is Prospero’s brother; he became the Duke of Milan by usurping the position from Prospero, who is determined to recover control of the territory. Sebastian is the younger brother of Alonso, the king of Sicily. Sebastian is presented as a parallel character to Antonio, who goads him to take over Sicily by murdering Alonso and his adviser Gonzalo; they are thwarted by magic. Both characters help Shakespeare emphasize the difference between legitimate and illegitimate accession to a political position.
Although Antonio is more evil, Sebastian is both greedy and cruel. When they are washed ashore, Alonso believes that his son and heir, Ferdinand, has perished at sea. Rather than offering encouragement that he may have survived or sympathy for his apparent loss, Sebastian blames Alonso because the reason for their voyage was to attend his daughter’s wedding, of which Sebastian disapproves.
Later, when Ariel and Prospero wreak enchantments on the shipwrecked men, they miss Antonio and Sebastian, who thus remain awake as the others sleep. Their villainy is established through their decision to kill Alonso and Gonzalo as they sleep. Antonio’s motive is to gain an ally who will free him from the tribute that he currently pays to Alonso. In his persuasive speeches to Sebastian, Antonio rationalizes that the takeover is their destiny (act 2, scene 1):
by that destiny to perform an act
Whereof what's past is prologue, what to come
In yours and my discharge.
When they draw their swords, however, another one of Ariel’s magic spells wakes up the sleeping men. The would-be assassins pretend to be loyal guardians. Intervening in their actions was not simply an ethical issue, however. The plot against Alonso was not in Prospero’s interest, because he wants Ferdinand, Alonso’s son, to marry Miranda, thereby uniting the two territories.