Dr. Koichi Matsudo is a supporting character in L change the WorLd. He is a scientist who was once the research partner of Dr. Kimihiko Nikaido. When the detective L is racing against time to recreate an antidote for a deadly engineered virus, he brings Nikaido's orphaned daughter Maki and a gifted young boy to Matsudo's laboratory for help. Matsudo is initially reluctant to involve himself in the case, cautious of the immense danger and the scale of the bioterrorist conspiracy, but he is persuaded by L's resolute determination. Through his scientific expertise, Matsudo makes a pivotal discovery: the virus relies on blood sugar to replicate, which explains why Maki, who has low blood sugar, shows no immediate signs of infection after being exposed. Recognizing the boy as the sole survivor of a village ravaged by the virus, Matsudo theorizes that his natural immunity could be the key to synthesizing a cure. His motivations are driven by a strong sense of medical ethics and a desire to honor the work of his late partner. Acting as the group's scientific anchor, he protects Maki while dedicating himself to the race for a treatment. Throughout the story, Matsudo develops from a hesitant outsider into a courageous and resourceful collaborator, ultimately succeeding in creating the antidote and playing a critical role in defeating the terrorist plot.