|This article is about a character in Higurashi no Naku Koro ni.|
This article or section contains untagged major spoilers for all of the original Higurashi no Naku Koro ni, possibly including the console-exclusive arcs. Readers who have not completed the main story are advised not to proceed further.
Koizumi is not given a sprite in the visual novels, though he is often described as an old man and in his advanced age has to use a wheelchair. Various adaptations give him a mustache and short brown hair while wearing suits.
Koizumi is a kindly man and treats people with respect and goodwill. However, he is the founder of a nationalist organization and supports eugenics, part of the reason he wanted to see Hinamizawa Syndrome research succeed. Koizumi wishes to see Japan grow into a major world power through weapons development and to that end he freely provides Takano with all the money necessary for her research.
Koizumi is a little perverted, teasing Takano with talk of touching her butt. He also shows some playboy tendencies, something noted by Takano during a meeting. The manga shows an intimidating side to Koizumi when he pulls a gun on her during their reunion and threatens her not to continue her grandfather's research, though he soon reveals it to be a test of resolve after seeing how determined Takano is.
Koizumi fought in World War II for the Japanese army and held the rank of Colonel. He became friends with the military surgeon Dr. Hifumi Takano after learning they came from the same hometown. Dr. Takano discovered the Hinamizawa Syndrome disease in some Japanese soldiers and believed it could turn the tide of the war, with Koizumi thinking the same way and offering his support. Although the request for research was denied due to the military's need for an instant-acting weapon, Koizumi still supported the doctor.
When World War II ended, Koizumi became a key figure in post-war restoration and improving the medical industry, all thanks to his political influence. Koizumi made a lot of connections, with many people referring to him as Koizumi-sensei. However, he and several other Japanese politicians were unsatisfied with the way things turned out and formed the secret organization Tokyo, aiming to fund weapons projects that were on par with nuclear weaponry and transform Japan into a major world power. Koizumi renewed the higher-ups' interest in Hinamizawa Syndrome and arranged for sponsors from the Self-Defense Force to inspect Dr. Takano's work, however he soon learned that the reason the military denied his research in the first place was because the syndrome was likely responsible for causing the Marco Polo Bridge Incident. The incident caused disputes between Japan and China as each side claimed the other was responsible, and the reveal of Hinamizawa Syndrome to the public would place the blame on Japan. Koizumi was barred from telling anyone of this, not even Dr. Takano, and he regretted that the doctor's hopes were crushed once more.
Sometime after Dr. Takano's death, his granddaughter Miyo Takano overtook his research. As the last remaining person who knew of the syndrome's secret, Koizumi approached her and revealed the real reason why the SDF refused the doctor's research. He offered Takano his unconditional support and funded her research, enough for her to open the Irie Clinic under his Alphabet Project. Koizumi also regularly paid Takano personally, letting her stockpile over 1 billion yen total.
In 1983 Koizumi suddenly passed away from a heart attack. This caused unrest in Tokyo, as Koizumi was the most influential member and a remnant of Japan's old generation. Several people from anti-Koizumi factions soon take his place and begin re-evaluating the projects he sponsored, including Hinamizawa Syndrome research.
Koizumi and Takano became friends during World War II and would always talk about war stories whenever they met after. Koizumi wholeheartedly wanted to see the doctor's research succeed but regretted that he couldn't help at all when he was rejected. After Dr. Takano's death, Koizumi hoped to make things right by supporting his granddaughter.
Koizumi has known Takano ever since she was a little girl. When Takano becomes an adult Koizumi starts teasing her more often, and she starts referring to her as Uncle. Koizumi personally gives Takano funding for Hinamizawa Syndrome research and other things, wanting to support her until the day he dies.
When the Emergency Manual is explained, a letter sent to Koizumi from Dr. Takano in 1945 is shown, where the doctor outlines the steps that must be taken should the queen carrier die.
Koizumi's involvement behind the scenes with Hinamizawa Syndrome research funding is shown. Sometime after Dr. Takano's death, Koizumi reunites with Miyo and hands her a case full of money, soon talking with her about Japan's past in abandoning nuclear missiles. He then reveals to her the real reason the SDF refused to fund Dr. Takano's research in the first place, being that it was responsible for triggering WWII and would cause foreign relations problems.
A flashback of Koizumi arranging sponsors for Dr. Takano's research is shown. He hoped to show how great Dr. Takano's research was but an elder told him not to get involved, informing him of the Marco Polo Bridge Incident. As such, Koizumi is not present two weeks later when the sponsors see the doctor's research and deride it.
Koizumi calls to congratulate Miyo when she makes progress in research and sends her a bottle of wine, the same kind her grandfather was saving for when his research would become recognized. Koizumi passes away a few years later, with Miyo attending his funeral and later struggling with the reduced budget the clinic receives because of the anti-Koizumi faction.
- Matsuribayashi Chapter 3.
- Matsuribayashi Chapter 7.
- Matsuribayashi Chapter 3. Narration: The woman who was helping [Koizumi] with his wheelchair was too beautiful to be a mere helper, so Takano had suspected something. As it turned out, her guess was correct. Although he was so old that he could die the next day, he wanted to stay sexually active until the very end.
- Festival Accompanying Arc Manga Volume 2, Chapter 6.
- Matsuribayashi Connecting Fragments 25: A roll of a one.
- Matsuribayashi Chapter 16.