Alonso is the King of Naples and the father of Ferdinand. He is actually the first of the principal cast to speak during the play, but the audience is given little to no information about him in the play’s opening scene, as The Tempest begins in media res, or in the middle of the action. The audience does not learn who Alonso is until the following scene, when Prospero delivers a lengthy monologue to Miranda outlining his fraught history with the men who have just been shipwrecked. He explains that Alonso aided Antonio in unseating Prospero as the Duke of Milan, which is why he and Miranda have been stranded on an island for the past twelve years.
Given Alonso’s treacherous past, it would be easy to assume that Alonso is the play’s antagonist. However, his character invokes more pity than contempt. Alonso is separated from his son during the shipwreck and spends the majority of the play mourning the child he assumes he has lost. He is practically debilitated from the guilt of believing he lives in his son’s place. He also blames himself for Ferdinand’s supposed death because the party was only traveling by sea to begin with because Alonso married his daughter off to the advantageous King of Tunis.
Alonso’s devotion to Ferdinand can be compared to Prospero’s devotion to Miranda. As a result, Alonso and Prospero can be interpreted as doubles, a common convention in the works of Shakespeare in which two characters mirror one another. Given their mutual love for their children (and their children’s impending marriage), it is no surprise that Prospero forgives Alonso after he expresses remorse and restores Prospero’s title.