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This article contains plot details from the latest books.
He has blond hair, piercing blue eyes, and a crescent-moon bite on his hand from the dog that he was jogging with when he tried to kidnap Sophie in the first book. It was also mentioned that his nose is now permanently crooked.
He was the jogger with a dog that tried to capture Sophie when she was living with her human family, he told her that the dog was his sister's and that was why it kept biting him. Luckily Mr. Forkle luckily stopped him and told Sophie to call the police if she ever saw him again. Sophie also notes that she couldn't hear any of his thoughts.
Gethen was held in captivity by the Black Swan. He had his fingernails frozen off by Squall to prevent the Neverseen from tracking his location, even though it was unnecessary because he was supposed to stay trapped in Lumenaria for his part of the timeline. To prevent his mind being broken, he found a way to tuck away his consciousness. When Sophie tried to probe his mind, it felt empty, like there was nothing there. He is also very good at blocking.
He was moved to the dungeons in Lumenaria, but later escaped with the Neverseen when he destroyed the castle. Through the months he was in the dungeons, Gethen accumulated enough mental power to bring down a huge structure using outward channeling. Throughout the book there was foreshadowing that he was exactly where he wanted to be. He stabbed Mr. Forkle when aiming for Oralie before his escape and managed to kill Mr. Forkle from blood loss.
To be added.