With a new series come new characters!

Humans

 Alvin the Treacherous : the arch-enemy of book's Hiccup arrives in the show with no different purpose, he's the main antagonist of the TV Series and leader of the Outcast Tribe. Determined to seek revenge on Berk for his banishing, he wishes to learn the Dragon Riders tecniques, in order to teach his fellow Outcasts to ride dragons as well. Unlike the book, he seems to own, once he learnt how to tame dragons, a Whispering Death. As the second season ends, he seems to have become allied of Berk.

 Dagur the Deranged: Hiccup's arch enemy in the TV Series, Dagur is the leader of the Berserkers, arrived in Berk to sign a contract to renew the peace between the Hooligans and his Berserkers. He came instead of his father, recently passed away, but spent his time to insult the latter and to search for dragons to kill. He later on allies with Alvin but betrays him to take control of his Outcasts, wanting revenge as well on Hiccup after his many defeatings from the Hoolingan's viking and his dragon riders. He doesn't have a dragon, the Skrill that he tamed in the second season was lost to Hiccup, enraging him even more.

 Mildew: Berk now lives in peace with dragons but not everyone are happy of these changings and Mildew, more than anyone, wants the dragons out of the village and keeps pressuring Stoick to get rid of them, at the point of poisoning them and by betraying his own village, becoming a spy for Alvin. Nobody in the village seems to stand Mildew, the only one that lives with him is his loyal sheeo, Fungus.

 Savage: every main villain has a second in command and Savage is Alvin's right hand. The Outcast uses a bone as his main weapon and it is him that does most of the dirty work, like holding Hiccup hostage, placing the traps and the Whispering Death's eggs' under the village of Berk.

 Bucket: it is unknown what his real name is and everyone call his this way because of the bucket that he always wears. His bucket seems to "feel" when a storm is arriving, as it contracts on poor Bucket's head giving him great headaches and letting him know that the weather is changing. The more pain he feels, the more strong the storm will be. He lives with Mulch and is a fisherman.

 Mulch: Bucket's best friend, they live together and take care of some animals useful to the village, such as chickens, yaks and sheep. Since Bucket is a little dumwitted, it is Mulch that tells him what to do and helps him in almost everything. He is a fisherman too.

 Gustav: Snotlout's small admirer, traded to him by his mother in exchange of what seemed a magical stone but, in truth, was a Changewing's egg. He idolized Snotlout at the point of dressing like him and acting like him. At some point Snotlout, sure that he was going to die, left Hookfang to the care of Gustav but, once he realized that his days weren't over, he took his dragon back. Gustav then, went into the forest and tamed a Monstrous Nightmare that he named Fanghook.

 Spitelout: briefly appeared in the first How to train your Dragon movie, Spitelout, in the TV series, was confirmed to be Snotlout's father. Spitelout asks a lot from his son as he wants Snotlout to be the best of everyone, making the young boy often insecure and scared of his father. He may also be Stoick's second in command.

 Johann the Trader: Berk surely can't have everything at disposition of the Vikings so, if anyone needs something specific, they can sure wait for Johann's ship to arrive in the village, to buy all that they need. Not only he exchange his goods with the Hooligans, he also is an useful ally against the attack of Alvin, since he too was a victing of the Outcast's madness, resulting in his ship being destroyed.

 Heather: this girl sure causes Astrid's jealousy since she begins to spend a lot of time with Hiccup. But her suspects are no wrong and the girl steals Stormfly along with the Book of Dragons. The riders finds out that she's a spy of Alvin and that the Outcast ordered her to go to Berk, become friends with the young riders and then, steal the Book. But Heather is no evil herself, she was forced to do so to save her parents.

Dragons

 Thornado: a powerful Thunderdrum, dragon of Stoick.

 Torch: this bay Typhoomerang causes Toothless' jealousy since Hiccup begins to take care of him a lot. But all this hatchling wants is to get back to his mother and siblings.