Mishima has short dark brown hair and brown eyes. He is regularly seen wearing a suit and tie.
Mishima is a skilled and dedicated investigator. He was recruited by Soichiro Yagami, and he becomes obsessed with his work with the Task Force, particularly in Light Yagami. Because of that, he sometimes is called "Death Note otaku".
His real name is Ryo Nakagami.
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, a fellow Death Note task force member 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, L's successor, to track down the new Death Note users. In the beginning, he and Ryuzaki capture Sakura Aoi, a Death Note user wreaking havoc at Shibuya-shi Sakamoto-chome and take possession of her Death Note after she dies of a heart attack. Later, they talk to the Death Note's Shinigami, Beppo, and he tells them that there are currently 6 Death Notes in the world.
When Yuki Shien, the Neo-Kira holds a private conversation with Ryuzaki, the Death Note Task Force tracks him down to home. At Kira's home, Matsuda find a Death Note page with his name written on it, and shoots himself while smiling, as instructed. This causes the Task Force to be dissolved.
Mishima listens to the conversation's recording between Ryuzaki and Kira, and getting a hint from that, he analyzes and edits the video of L that Ryuzaki had posted earlier, and discovers him to be an owner as well. After a heated fight and debate, Mishima decides to trust Ryuzaki and his shinigami Arma.
However, taking precaution, Ryuzaki implicates Mishima and has him jailed. Ryuzaki later changes his mind, and breaks him out. Together, they talk with Kira in order to set up a meeting, where they will steal his 4 Death Notes. When the servers get hacked, Nanase, destroys the scrambler. However, that turns out to be a distraction, as Misa Amane shows up and writes Ryuzaki's, as well as Kuromoto's and Uragami's, 2 Death Note Task Force member's names in her Death Note.
Angered by this, Mishima follows Yuki to the Aro Mountain Resort, where he plans to threaten him into giving all the Death Notes to him. However, Ryuzaki is revealed to be alive, and he deduces that someone has already written his name for a later date. Ryuzaki concludes that the only people near him were the members of the Death Note Task Force, and accuses Mishima of being the real Kira. When Mishima denies this, Ryuzaki forces him to touch the Death Notes.
Mishima is then seen to be having a flashback to Teru Mikami in the woods, killing Light Yagami's child. Before Mikami could write his name, Mishima shoots him and kills him. He then takes his Death Note for himself, and is coerced by Ryuk, and the powers of the Death Note to become Kira. He writes Ryuzaki's name for December 25th, which has not arrived yet, revealing why he is not dead yet. Mishima, then gave up his Death Note, and also a video of Light Yagami made for his child, which he edits into a plea for help, and an announcement for the world. The Death Note then is given to Yuki Shien, who becomes the next Kira.
After the end of the flashback, Ryuzaki puts a gun to Mishima's head. Yuki starts to write his name on a scrap of paper in his watch, but before he could finish, the police arrives to kill all 3 of them. Yuki is shot, and he dies, giving all of his Death Notes to Mishima, as to keep them out of the government's hands.
Mishima and Ryuzaki escape through a tunnel, where they meet Nanase, who, having heard all of the conversation between those 3 in building, is insanely angry at her. She reveals that her brother, her only family, was a criminal who was killed by Kira. However Arma shows up, and saves the 2 of them by killing Nanase, dying in the process. Outside the tunnel, the police captures them and they are taken for execution.
On his way to prison, Ryuk meets Ryuzaki one last time before leaving. He tells him that the reason why the shinigamis are so desperately looking for Kira's successor is because the shinigami who does the job will be given the title of Shinigami King, when the current one retires. He reveals that as soon as the Death Notes will be destroyed, more will be send by the shinigamis, until Kira's successor will be found. Ryuk then leaves for the Shinigami world, citing that his fun will therefore not be over for a long time.
Ryuzaki later visits Mishima in his jail cell. He tells him that the Death Note courier squad was attacked by terrorists, and that 4 of them have been burned, and then grants him his title of 'L', revealing that he knows no one more capable of tracking down the Death Notes that have fallen to Earth. Mishima is released from his cell, and Ryuzaki dies of a heart attack in the cell.