Mishima has short dark brown hair and brown eyes. He is regularly seen wearing a suit and tie.
In the first episode of the miniseries Death Note: New Generation, Mishima is brought into the Task Force by Touta Matsuda after passing a polygraph test. Upon joining, he learns about the Death Notes and the truth of the Kira case. He develops a deep interest in Light Yagami in particular, and diligently researches all the available information. Fellow task member Sho Nanase begins calling him "Death Note Otaku" because of his dedication.
When it appears that a criminal has died of a heart attack, suggesting a return of Kira, Mishima and Nanase investigate. Mishima realizes that a local flower could be used as poison and mimic the appearance of a heart attack, which leads to an arrest. The information leaks to the press, and an unseen Death Note user controls the suspect to announce the return of Kira. Criminals begin dying of heart attacks again, and the Task Force realizes that they'll be in danger. Mishima, along with all the other members, decides to stay on to find the new Kira.
In the film Death Note: Light Up the NEW World, Mishima is central as a Task Force member searching for the six Death Notes. He teams up with Ryuzaki to track down the new Death Note users, most notably Sakura Aoi and Yuki Shien.