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Kozo Fuyutsuki (冬月コウゾウ, "Fuyutsuki Kōzō") is a fictional character from the Neon Genesis Evangelion franchise. He is second-in-command of NERV HQ and Gendo Ikari's right hand man. As Deputy Commander of NERV, Fuyutsuki frequently takes command of situations at the base whenever Gendo is away on business. Gendo also tends to unload the more mundane work, such as town council meetings, paperwork, and the like, onto Fuyutsuki.
Prior to Second Impact, Fuyutsuki was a professor of metaphysical biology at Kyoto University. While there, he met the young bioengineer student Yui Ikari and became her "sensei". The exact nature of their academic relationship is unknown, although they were close and spent time together off campus. Fuyutsuki was smitten with "Yui-kun" from the start, although he kept this entirely to himself.
Not long after, a man named Gendo Rokubungi, who was being held at the Kyoto Prefecture Police Department after a bar fight, asked to be released into Fuyutsuki's custody. He complied, although he found Rokubungi distasteful. Yui later introduced Gendo to Fuyutsuki formerly, and then, during a hike, revealed that she was actually dating him—news that left her sensei visibly disconcerted despite efforts to conceal his own reaction.
This was a strong suggestion that Fuyutsuki was attracted to Yui Ikari, as well as being impressed with her work. At the very least, Fuytusuki appeared to share some of Yui’s admiration for humanity. In Episode 12, during a philosophical debate with Gendo at the South Pole Fuyutsuki stated that he’d "prefer a world where people live, no matter how stained with sin". Also, he has said that he agrees with Yui’s ideas more than SEELE’s.
But on a personal level, Fuyutsuki was said to want little to do with people in his earlier life 
Fuyutsuki survived Second Impact, but his career as a university professor came to an end. In 2002, he "set up shop" at the ruins of Toyohashi (in Aichi Prefecture, east of Kyoto) as an unlicensed doctor to compensate for the shortage being experienced everywhere. Fuyutsuki was selected to be part of the United Nations' official investigatory expedition to the South Pole (on the basis on an anonymous recommendation by Gendo). This eventually leads him back to Gendo Ikari, director of the Artificial Evolution Laboratory. During this time he also finds Gendo and Yui to be married, and with child. Although Kōzō's inquiry is cut short, the incident and it's subsequent cover-up becomes an obsession for him, and he continues to investigate on his own.
He then confronted Gendo at the Artificial Evolution Laboratory with his findings. Gendo, unmoved by Fuyutsuki's threats of taking the information public, took him down into the GeoFront and the under-construction facilities. Upon showing him the prototype Eva unit, he invited Fuyutsuki to "join us in creating a new genesis for Mankind".
Fuyutsuki subsequently joined Gehirn, though it would appear that he had little choice in the matter. He confides in Yui that he was under threat of erasure by SEELE for knowing too much, and "working for the enemy" would provide a means to avoid that fate.
During the present of the series, Fuyutsuki is nearly always seen in the company of Gendo Ikari, but it is clear that their alliance is one of convenience and business. The two men do achieve some unity in that both are interested in seeing Yui again, however. It is unclear whether Gendo or Yui knew of Fuyutsuki’s attraction to her.
At NERV, Fuyutsuki projects considerably less harsh of a personality than Gendo, confirmed when, after the disaster that was the initial attack on Israfel, Ritsuko Akagi told Misato Katsuragi that if Gendo hadn’t been elsewhere and the (still very angry) Fuyutsuki wasn’t in full charge of the operation, Misato would have been fired already. His usually deadpan observations often provide a source of very dry humour. His exchanges with Gendo often serve as a conscience stimulator for the Commander, often leading him to reveal a little of his plans to the audience. He is tall—taller than Gendo—and always wears a dark reddish-brown buttonless uniform; this coupled with his stern yet calm, stoic demeanor makes him an imposing figure. If he is in command during combat, however, he projects a much more active style than Gendo's utter calm.
Fuyutsuki does not particularly like Gendo. His history with him, as detailed above (and in Episode 21), is rather rocky. It seems, partially by his own admission, that he follows Gendo's plans not only for the sake of humanity but also for Yui's. He respects Gendo's ability to lead and persuade despite, in return, bearing the brunt of the "trivial duties" at NERV. Gendo respects Fuyutsuki enough to leave him in charge of NERV in his absence, or when he needs to deal with an issue personally. He alone appears to be aware of the extent Gendo's intentions with the Human Instrumentality Project, though he himself is not fully in the know. His prodding of Gendo often results in the latter revealing a marginal detail or two to the audience. Other than Gendo—though as a result of Gendo's influence—Fuyutsuki is the only one of the NERV staff to have had dealings with SEELE before the advent of Kaji and the interrogation of Ritsuko.
Fuyutsuki was smitten with Yui from the day he met her; though the attraction seemed purely platonic and academic at first, it is implied that he developed feelings for her. She had submitted a paper to him for review, and in their discussion afterward he seemed dismayed that Yui would choose to start a family and interrupt what was promising to be a brilliant career. He was even more perturbed when she started dating Gendo Rokubungi, of whom he did not think highly, and could not contain his surprise (and displeasure) when Yui revealed they were to marry. Yui considered Fuyutsuki her sensei, and while it is unknown how much she knew of SEELE's intentions, she was pleased when he joined Gehirn. Her last words, regarding Shinji, were spoken to her sensei. During Instrumentality, Rei/Lilith turns into Yui, and he embraces her before being reverted to LCL.
The Evangelion pilots
Fuyutsuki spends little time interacting with the pilots themselves. He is aware of Shinji's story, and perhaps due to the boy being Yui's son he harbours something of a soft spot for him (though almost never outwardly). He is aware of Gendo's attachment to Rei, and due to his involvement in the Human Instrumentality Project he is well aware of Rei's origins, though they almost never interact outside of combat situations. Fuyutsuki often comically cringes at Asuka and Shinji's public bickering as he feels they are humiliating NERV; Asuka herself felt humiliated during her and Shinji's dressing down by Fuyutsuki after the first failed sortie against Israfel. He is never shown to have interacted with Toji or Kaworu, though he too appeared to know about Kaworu's provenance.
The NERV staff respect Fuyutsuki, not just for his leadership and stoicism, but also for his humanity in contrast to Gendo's cold indifference. He is less severe than Gendo—something Misato turns out to be grateful for after the first attempt against Israfel failed; Ritsuko comments that had it not been for the "lucky circumstance" that Gendo was away and Fuyutsuki in charge, Misato would have been fired. Ritsuko is the only other NERV staff member who is in the know about much of the Human Instrumentality Project, the secrets of NERV, and has access to Terminal Dogma; as such, she shares some of Fuyutsuki's knowledge and impressions. He always appears to keep an eye on Ritsuko's experiments (as in Episode 11). She, like Misato, usually defers to him. Maya Ibuki comments to Ritsuko on the stoic way Fuyutsuki and Gendo are taking the rising heat after NERV HQ loses power (and thus air conditioning); she is unaware that Fuyutsuki has his feet in a bucket of water.
In other media
Neon Genesis Evangelion -ANIMA-
In Evangelion ANIMA, Fuyutsuki went to jail because of his involvement in the Human Instrumentality Project, but was soon after released because of his health. He returned to NERV JAPAN to help when Misato got abducted by Rei Quatre.
- "Fuyutsuki" literally means "Winter Moon", and was the name of a Akitsuki-class destroyer of the Japanese Imperial Navy from World War II. The Destroyer Fuyutsuki rescued the survivors of the Japanese battleship Yamato after it was sunk by American aircraft in Japanese's Operation Ten-go.
- Fuyutsuki's rank has been alternatively translated as "Vice Commander", "Deputy Commander", or "Sub-Commander", etc. The original Japanese word is fuku (as in fukushirei), and all of these alternate translations mean pretty much the same thing. Evapedia consistently uses "Vice Commander" for convenience.
- Fuyutsuki is named after a Japanese warship. His design comes from Col. Alec Freeman of the TV series UFO. In Evangelion Proposal, he was originally going to be slightly younger than Gendo. It also ascribed him "Quiet acceptance of Misato’s reckless strategies due largely to his personality."