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</PRE></noinclude>Koizumi is an old military officer who's a friend of Hifumi whom he met during the WWII. Hifumi informed him of his studies into the Hinamizawa Syndrome, and Koizumi supported his research. However, due to pressure from forces in the government, he allows Hifumi's research to be ridiculed and effectively halted. After Hifumi's death, he decides to financially and politically support Miyo to continue Hifumi's research by way of contrition. Due to his support, she attracts the attention of political groups such as "Tokyo" as well as the government as a whole. She receives a major's commission and a major research laboratory disguised as the Irie Clinic. His death removes his personal support, and the major government faction prefers to quietly cure the disease then quash any knowledge of it.

Koizumi met Takano Hifumi during WWII and the two have been close friends ever since. Being a secondary father figure to Miyo, he establishes an opportunity for Hifumi present to his research on the Hinamizawa Syndrome before high-ranking officials in order to receive funding. However, Koizumi suspiciously does not attend the meeting and the officials utterly humiliate Hifumi, calling his work "delusional" and "fictional" after having read his thesis and convince him to give up on the research; denying the existence of any such illness or parasite. It is later revealed that Koizumi was too guilty to be present at the meeting as he already knew the outcome. These events leave both Miyo and Hifumi heartbroken and essentially inspire Miyo to become a talented researcher and someday take revenge on the people who laughed at her grandfather's work by having them acknowledge it.

Koizumi approaches Takano Miyo later in her adulthood as she is trying to find the appropriate connections to re-establish her grandfather's research. He is extremely fond of Miyo, having known her since childhood as Hifumi's adopted granddaughter and grows increasingly impressed by her academic achievements and determination to undertake Hifumi's research, even after his untimely suicide. Moved by her iron will and his own regret to help Hifumi, he financially assists Miyo with her research and helps her establish the Irie Clinic under the influence of "Tokyo".

The manga adaptation greatly expands on his presence when compared to the ''anime. In the manga'' adaptation, Koizumi organises a meeting with Takano where he threatens her with a gun to halt her research. He claims that her and Hifumi's work, "would wake up the ghosts of history." When she demands to know why the government effectively stopped Hifumi's research, Koizumi explains that during the war Hifumi discovered, "that soldiers from Hinamizawa were responsible for most of the derangement cases," and he started his research into what he named the "Hinamizawa Syndrome"

Koizumi tells her about the Marco Polo Bridge incident (July, 7th, Showa 12 [1937]) in Beijing) where the Japanese army was shot at during training which degenerated into combat between the Japanese and Chinese armies, with both claiming the other initiated the conflict. Koizumi reveals that in the Japanese army at this bridge, there was a person from Hinamizawa. The implication is clear: a Japanese soldier may have started the conflict the Japanese government used to justify invasion which eventually led to Japan's involvement in WWII. The government always denied Japanese responsibility for the conflct.

Regarding who initiated the incident, Koizumi shouts: "They don't know the truth about it!" He then asks her to stop, then tells her that this research will put her in danger. She stands up and refuses, even at gunpoint. Impressed, Koizumi informs her that one bullet from his gun will effectively end the research. She tells him to shoot, but also tells him that his bullet certainly will not hit her.

He starts to laugh and remarks that, "Takano-kun was really blessed with a nice grandchild!" While she is stunned by his change, he explains that he needed to test her determination. He then promises to fully support her. He finally asks her to, "help succeed Takano-kun's will and complete the research on Hinamizawa Syndrome".

In Matsuribayashi-hen, Koizumi's death before Showa 58 weakens the government's interest in the Hinamizawa Syndrome. It is revealed that many in power do not favor of the research. In light of the UN Biological Weapons Convention, "Tokyo" concludes that secret research on bacteriological weapons would tarnish Japan's international image, and that decide that their funding could be invested more usefully. Takano desperately tries to convince them about the importance of the research and the revolutionary effect it would have on the scientific community, but she suffers humiliation similar to her grandfather when they declare her previously lauded theories to be too farfetched. They ultimately order her to finish her research, develop a proper cure for the illness within three years, and then disband the Irie Institution and destroy all evidence of the research. These events psychologically corner Takano and motivate her to become pivotal in "Tokyo's" political conspiracy, which ultimately results in the Great Hinamizawa Disaster.

Koizumi has two outfits, a black suit and another with a green shirt. He has brown hair, eyes and a moustache. In the manga he also wears a white hat.

pt-br:Koizumi Koizumi