Kensuke Mitsui appears across multiple installments of the series. As a 27-year-old office worker in the main storyline, he conceals his otaku interests from coworkers while maintaining a close, enduring friendship with Hirotaka Nifuji since childhood. Kensuke actively supports Hirotaka with personal matters and engages competitively in online gaming alongside Hirotaka and their friends. He stands 172 cm tall. His personality combines shyness and introversion, particularly regarding his unspoken romantic feelings for Narumi Momose, though he remains attentive to her needs. Kensuke demonstrates responsibility, detail orientation, and a strong preference for routine and logical thinking, often struggling with unexpected changes and fixating on past details. Expanded media, including OVAs and spin-offs, depict a younger Kensuke at 19 during his university years. Studying on the same campus as Naoya Nifuji, he maintains childhood friendships with Naoya and Yoshiki Ichijou. Nicknamed "Ken-chan" by Naoya, this younger Kensuke interacts with classmates like Kō Sakuragi, exhibiting a more energetic and occasionally provocative personality contrasting his reserved adult self. Kensuke's background includes friendships originating in kindergarten with Naoya Nifuji. His interactions span workplace colleagues and university peers. Across all depictions, consistent traits emerge: deep loyalty in friendships, a passion for gaming, and challenges navigating personal relationships, especially unspoken romantic feelings. While his portrayal varies slightly between the more forceful younger self and the methodical adult, his core dedication to friendships and hobbies remains constant.