Berthel attended Berk's 400 year anniversary with Amos, for the sole purpose of finding Hiccup and capturing him. They watch and wait quietly in the shadows for their chance to strike. Due to the festivities, they had to wait until Berk's merry making came to an end and the villagers retired to their homes. Though the actual kidnapping is not seen, it's implied that they snuck into Stoick's house while Hiccup and his father were sleeping, and abducted the younger man, knocking him out first to keep him quiet.
They bring the unconscious Hiccup to their small ship, and sail off into the night for Sleipnir Island where they are to collect the bounty. After daybreak, the two go to check on their prisoner whom they left below deck, only to see he is awake and trying to cut himself free with a nail in the ship's hull. Amos suggests Berthel knock Hiccup out again, but Hiccup convinces them not to because they might accidentally kill him. So Amos asks Berthel if the Wanted Poster stated if Viggo wanted him alive or dead, to which Berthel holds up two hook prosthetics for hands and says that holding things isn't his strong suit.
Just then, footsteps can be heard up on deck, and Hiccup reminds Amos and Berthel that his friends and their dragons will come save him. In terror at the thought of dragons, both older men run up to the deck, only to be met by Savage rather than the Riders and their dragons. Savage then throws them both overboard so he can take Hiccup, and the bounty, for himself. After tossing Hiccup into his own ship, Savage sails by Amos and Berthel as they cling to each other in the water, mockingly thanking them for their help. Shivering, Berthel tells Amos to hold him.
They, however, manage to swim back to their own ship and, with the help of Berthel's hooks for hands, climb back onto the deck, only to be met by Stoick and the Riders. They confess that they no longer have Hiccup and that Savage came and took him away. Ruff and Tuff quickly interrogate the two on the bounty the twins had put on themselves, and ask about the Wanted Posters they found on a roll in the ship's outhouse. Amos confesses that the paper they used for the flyers was soothing, and thus works great as "outhouse tissue". The twins agree and laugh, and in frustration, Stoick throws both Amos and Berthel back overboard. Ruff calls after them that she'll have her people contact their people concerning the twins' bounty.
Berthel is seen briefly in "Sandbusted". He is still working with Amos, as well as other unnamed people. Amos steals Hiccup's Betrothal Gift to Astrid and it gets handed off several times to other thieves, including Berthel, to confuse Hiccup.
Berthel is shown to be the less intelligent partner amongst the two. He is also forgetful (although that might be contributed to his two hooks for hands). He also seems to be very loyal to Amos, doing what he says with no hesitation.
Berthel wears a simple brown tunic over a green shirt and wears a large belt around his waist with studs all around. He also has two hooks for hands.