He and Seth barely get along, and he tends to brag about the trips and places he goes to with his family, and often makes fun of Seth's breath, and Seth in turn makes fun of Knox's socks.
He unknowingly plays a key role in saving Wyrmroost by retrieving a scepter to reinforce the magical protections of the Blackwell Keep. He did so believing the entire experience was some kind of virtual reality game but was completely ignorant of the magical creatures he'd interacted with, and the dragons he'd inadvertently thwarted.
In Wrath of the Dragon King, Knox suspects his "virtual reality" experience was something else entirely, when he finds his sister Tessa playing with butterflies and saying they are fairies, and mentions a milk they like to drink. On a dare, Knox drinks the milk and discovers the truth of Fablehaven after Grandma and Grandpa Larsen explain everything to him, including the truth of what he really did at Wyrmroost.
Curious about Wyrmroost and Seth, Knox steals the keys to the dungeon and is joined by Tessa after she threatens to tattle. They meet the goblin Slaggo who helps them find the barrel to Wyrmroost but suggests they leave the dungeon keys with him.
They make it through the barrel and arrive in the Black Keep where they meet with Grandma and Grandpa Sorenson. Before they can be sent back however, the barrel disappears and the goblin that was watching it is murdered.