Norbert the Nutjob is the main antagonist of two of the How to Train Your Dragon book series, How to Cheat a Dragon's Curse and How to Ride a Dragon's Storm.
How to Cheat a Dragon's Curse
Hiccup's Grandfather, Old Wrinkly said that the only cure for Vorpentitis is the potato. The father of Norbert the Nutjob got it from America, but he died of being killed by the Doomfang. Hiccup will steal the potato to cure Fishlegs from Vorpentitis. Hiccup and Camicazi went to the hall of the Hysteric Tribe and stole the potato. When they went to the sea, the Doomfang got the potato and it turned out to have Vorpentitis and got cured. When both Hiccup and Camicazi went back, Fishlegs did not have Vorpentitis and it turned out to be Hiccup. He got cured by the arrow from the potato.
How to Ride a Dragon's Storm
Hiccup, Fishlegs, and Camicazi got caught by Norbert the Nutjob. They are planning to go to America. When they are near America, the sea dragon, Leviathorgan attacked the ship. Hiccup and Norbert fought and reached the top of the boat. Hiccup dropped into the water and Norbert got struck by lightning and it also affected the dragon.
How to Fight a Dragon's Fury
Norbert was revealed to have survived, as he was in the audience at Hiccup's congregation as King of the Wilderwest.
Norbert the Nutjob appeared to be similar to Two-Face. Half of his face looked different. One of his eyes appeared bigger than the other one. He also has a two sided axe. One is rusty while the other one is gold. He used it as random fates. One is to live and the other one is to die. This is similar to the coin-flip of Two-Face.
Norbert the Nutjob is likely psychopath. He is also hateful. When Hiccup stole the potato, Norbert wanted to kill him. Few years later, Norbert saw Hiccup again and wanted to kill him, but failed the attempt.