|“||If we're not careful, this island will become as treacherous as the rest of the world!||”|
— Alonso, The End
Alonso is one of the islanders that the Baudelaires meet in The End.
Alonso washed up on The Island after enduring a terrible storm and a dreadful political scandal involving a man named Gonzalo. He wished to leave his life behind and have a peaceful existence, so he joined the colony and became very loyal to Ishmael. His job in the colony seemed to be retrieving seaweed from the ocean with Ariel.
While storm-scavenging, Alonso finds the propeller of an airplane, theorizing that a pilot must have flown into the storm. He brings it to Ishmael, who suggests he throw it to the arboretum. Though Violet Baudelaire, a new castaway, suggests using the propeller to make a fan, an idea Alonso likes, he eventually follows Ishmael's advice, after Ishmael points out that they could potentially fight over who stands by the fan.
The next day, on Decision Day, a mutiny breaks out, and Alonso fights Willa until Ishmael arrives. He informs Ishmael of the mutiny and schism.
When Olaf arrives, telling a story, he mentions Gonzalo while Olaf is alluding to somebody Monday blackmailed. When the Medusoid Mycelium, he turns on Ishmael for the first time, as it was revealed that Ishmael was not disabled at all. He claims Ishmael's dishonesty was the root of their problems. However, when Ishmael suggests they drink cordial to dilute the poison, he does so, which drugs him into obedience. Ishmael gathers him and the other islanders onto the outrigger, convincing them that the Baudelaires were the root of the trouble, and they set off for Lousy Lane, though they will not make it in time.
It is unknown if he met his fate by being poisoned by the Medusoid Mycelium. However, it is noted that the Incredibly Deadly Viper followed the boat with an apple, and as the Baudelaires did not know this, it is possible some islanders survived to report this to Lemony Snicket. Friday knew the snake to be friendly, and thus may have taken the apple from it; however, it is unknown if she would have been able to convince Alonso or the drugged islanders to eat it.
Netflix Series Divergent Canon
In the Netflix series, Alonso's backstory is not mentioned, and he is now the island's cook, instead of Miranda Caliban. He mentions washing up on the island in a tempest.
He finds a large birdcage while scavenging, which Ishmael suggests he get rid of. However, Ishmael later decides to use the cage to trap Count Olaf. When he shows the Baudelaires around, Alonso shows them the plain dishes he prepares. He is interested when Violet and Sunny mention inventions and spices.
He is on the beach for Decision Day, and views Kit arrive and Olaf poison them. He sails off with the other islanders. He may have survived, as the Incredibly Deadly Viper is seen delivering an apple to the boat. 
He is portrayed by Simon Chin.
- He is named after King Alonso of Naples from William Shakespeare's The Tempest.
- The play concerns a number of people shipwrecked on an island, much like the colonists.
- Alonso's mention of a "dreadful political scandal" in his past involving a man named Gonzalo is likely also a reference to the play; in the play, Alonso allowed Antonio to usurp the dukedom of his brother, Prospero, and send Prospero and Miranda to their deaths at sea. However, an old councillor, Gonzalo, gave them enough supplies to make it to an island.
- Ariel, Ferdinand and Miranda Caliban also derive their names from the play. (In Miranda's case, also her surname.) Count Olaf also uses Stephano as a name for a disguise.