He was Japanese light novel and game scenario author from Ibaraki Prefecture, Japan. He was well known for being the author of the Zero no Tsukaima light novels and works in Front Wing game studio.

In July 2011, he revealed on Media Factory's website that he had advanced stage cancer that was discovered in February of that year and which was untreatable at the time, which affected his work on the final two volumes of Zero no Tsukaima. After an unrelated gallstone surgery, the cancerous growth was found to have shrunk allowing surgery to take place in early August 2011.He was later readmitted into the hospital in December 2011, and had another surgery in November 2012.
Yamaguchi died on April 4, 2013 at age 41.