Alonso, the King of Naples and father of Ferdinand, is the enemy of Prospero because because he helped Antonio overthrow Prospero as Duke of Milan. Alonso is returning from the wedding of his daughter when his ship is caught in a tempest and he is stranded upon an island. The tempest was engineered by Prospero so that he could extract his revenge upon Alonso.
Gonzalo, who among the characters stuck on the island after the storm, is a trusted advisor to King Alonso. Gonzalo is honorable and optimistic, and saves Alonso from from being killed by Sebastian and Antonio. Because of his goodness, he is the only one on the island whom Prospero spares from madness.
Antonio is the true villain of the play. Greedy and ambitious, he overthrows his brother Prospero from the throne of Milan and exiles the former Duke on a remote island with his infant daughter. Although Prospero forgives him in the end, Antonio remains unrepentant.
Alonso is the King of Naples at the time of the play and the father of Ferdinand, who will become Miranda's husband. He has been involved in the overthrow and banishment of Prospero and Miranda, but after being shipwrecked on the island due to Prospero's magic, he is worn down by grief at the supposed drowning of his son and becomes repentant. Prospero forgives him, and looks forward to their final reconciliation in the union of their children. For more detail, see the first link below.
Gonzalo is the most positive figure (apart from Ferdinand) of those shipwrecked. He is Alonso's adviser, and is moral, honest, and consistently optimistic. It was he who provided Prospero and the infant Miranda with the supplies they needed to survive when they were banished from Milan, including the magic books that are the source of Prospero's present powers. Because of this, Prospero treats Gonzalo more gently than the others involved in his banishment, and commends his morality at the end. For more detail, see the second link below.
Antonio has become Duke of Milan by betraying and exiling his brother, Prospero. He is presented as an immoral and foul-tempered character constantly abusing other people and even suggesting murder. He is thus punished severely by Prospero, and when Prospero relents at the end and pardons him, there remains a very real question as to whether he is truly repentant. For more detail, see the third link below.
alonso is the king of naples . antonio is prosperos brother and gonzalo is prosperos right hand-man