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Koichi Matsudo (松戸 浩一, Matsudo Kōichi) is a scientist and former colleague of Professor Nikaido.
Koichi is an older man with a short crop of black hair. He wears a white uniform and gold-colored glasses. He has noticeable stubble around his mouth and chin.
When Maki Nikaido is infected by the virus, L takes her to see Matsudo, whose name he remembers as a co-writer of a book with Professor Nikaido. Matsudo scolds L for bringing Maki to him in her deteriorating condition, saying that he had put countless lives at risk by doing so. L asks Matsudo to help create an antidote for the virus, which, at first, Matsudo is dead set against, explaining that he had made a vaccination once before that had ended up killing people. L points out that Professor Nikaido had been successful, and so Matsudo relents, agreeing to help find an antidote.
Matsudo eventually realizes that Maki's supposed immunity to the virus was actually due to her low blood sugar, explaining that the virus needed sufficient energy to be able to infect her. Maki was indeed infected, but the virus had so far remained dorment. Matsudo goes on to say that this means Maki is not the key to a cure, as blood sugar is important for humans to function, and that he could not use this as a method to stop the virus.
Things appear to be going nowhere, when the boy solves an equation he had written out on a chalkboard. He begins chanting the answer to the problem, "13:11". Matsudo unsuccessfully tries to get him to stop, and L soon realizes that Near is trying to tell them something. Maki says that she recognizes the equation as part of her homework that her father gave her, leading L to believe that there was some sort of hidden message in the equation.
The answer, "13:11" equals "MK" the thirteenth and eleventh letters of the alphabet, which they concluded stood for Midkine, a particular protein which Matsudo explained must be the key to the antidote. Noting the boy who appeared to be immune to the virus, he states that the child likely has high levels of Midkine in his body, explaining his apparent immunity. Taking blood samples from the boy, Matsudo is eventually able to create a successful antidote, saving numerous lives by doing so.