Alvin the Treacherous

Introduced in the Riders of Berk episode Alvin and the Outcasts. He's the leader of a group of Vikings who were banished by Stoick from Berk for unknown reasons called "The Outcasts".

Alvin, alongside his followers, live on a remote island that is still being plagued by dragon raids, and Alvin's goal was to capture the fierce "Dragon Conqueror" of Berk so that he could rid his island of dragons, only to find that the "dragon conqueror" was Hiccup.

After seeing that Hiccup doesn't conqueror dragons, but rides them, Alvin's intentions switch on capturing Hiccup to gain his knowledge on riding dragons for his own evil intentions, eventually wanting to not only rid his island of dragons, but use the dragons he'll eventually train to attack and regain Berk.

It seems that Alvin and Stoick had a long time rivalry that spans before Alvin was even banished, and it's implied that Alvin took something or someone that was valuable to Stoick in the past.

Despite Alvin not exactly feeling he's a bad person, he does resort to immediate capture and violence to get what he wants. In Alvin and the Outcasts he seemed to have his interrogation and capturing sights on Astrid.

Alvin makes his return in the episode Heather Report, Part 1 when he sends a spy--Heather--to Berk to retrieve knowledge of the dragons, and is eventually able to get the Book of Dragons because of her.

In Heather Report, Part 2 Alvin and his followers have the book in their custody and use it in the mock-version of the Kill Ring, with the assistance of Astrid--whom they think is Heather due to Astrid being in disguise--and quickly find the book to be helpful in the ways they want.

Though when Astrid tries to make off with the book, and Heather's parents, Alvin is quick to figure out that it's all a ploy and uses Astrid as bait to find Hiccup. He does, though, offer Hiccup a chance to save himself by asking him to join him as "his knowledge of dragons" and Alvin's own knowledge of warfare would make a good team. Hiccup, of course, turned him down.

Alvin eventually lose the book even though he used Astrid as bait again to get Hiccup to surrender or he'd throw Astrid off a cliff, but after Astrid was saved by a Nightmare she made friends with while on the island Alvin immediately saw that a dragons bonding with a human was something the book would never teach them, and fell back on his first plan to simply capture Hiccup.

in Defiant One Alvin is, once again, thwarted in his run of the mill plan to capture Hiccup when Hiccup and Snotlout accidentally got stranded on Outcast Island. After being defeated one time too many, Alvin stated that he'd get Hiccup even if he had to go get him himself. This plan seems to have come into effect in We are Family, part 1 when he forms an alliance with Mildew to get Hiccup and Toothless captured--forging notes into Bork's archives of a island filled with night furies to get Hiccup and Toothless alone.