Tsuya Yajin is a 15-year-old part-time worker stationed at a lunar colony on Earth's Moon. Of Chinese descent, he assists in maintaining and fine-tuning weapon systems for warships operating from the moonbase, including the vessel Queen Fuji. Aboard the Queen Fuji, his primary responsibility involves operating the warship's weapons systems, where he displays sharp marksmanship. Yajin exhibits a sarcastic demeanor and is documented deliberately providing false information to Iczer-3 on multiple occasions. He develops a significant romantic interest in fellow Queen Fuji crew member Nagisa Kasumi. This motivates protective actions, such as confining Iczer-3 to the brig after Iczer-3 summoned the Iczer-Robo during battle, believing Nagisa was endangered.

Later developments reveal Yajin was born in Osaka, Japan. His parents operated a well-regarded Chinese restaurant, and he possesses notable culinary skills, demonstrated during an impromptu cooking competition with Atros in the Iczer-3 CD dramas. While his romantic feelings initially centered on Nagisa Kasumi, his relationship evolved over time, leading him to date Kawai Shizuka, Nagisa's best friend and co-worker, during the events chronicled in the "Iczelion" novelization.