Kōichi Shidōu (紫藤浩一, Shidōu Kōichi) is the main antagonist of Highschool of the Dead. He is the leader of ''Them'' and the teacher of Class 3A.
He is Class 3A's teacher. Rei Miyamoto is afraid of and disgusted by him. Saeko Busujima says his name with a scowl, showing that he apparently is a villainous character. Both girls have good reasons to do so as we see him leaving behind a student who has sprained his ankle and even kicking him back towards a group of zombies chasing them. He then makes a speech about how weaklings do not deserve to exist.
Shido is very attractive; depicted wearing a black pinstripe suit. Inside the suit, he wears a white dress shirt along with a yellow tie. He also wears black-framed glasses. Shido's body frame is very thin and he has various expressions seen throughout the series. This includes switching from a peaceful expression to one with intense hate and scorn. His outwardly appearance is that of a fragile man, and this appearance often aids him in gaining undeserved trust from various characters throughout the series who are not aware of his debauchery and evil intentions.
Koichi Shido is the son of a prominant politician, Ichirou Shido. When he was a child, Koichi greatly admired his father, the latter being a diet debater as well as a canidate for cabinent minister. However, while Ichirou's public life made him seem like a noble man, his life at home was the exact opposite. Koichi constantly heard arguments between his parents, one such fight revealed that Ichirou was commiting adultery on his wife. Mrs. Shido began to drink heavily, which caused her to die of alcohol poisoning when Koichi was in high school. Despite this, Koichi still trusted his father even then. He left home and continued his studies at a university that his father had been director of before his ascention into politics.
After Koichi eventually returned to Tokonosu, he became a teacher at Fujimi Academy, wanting to ensure a future for the next generation. He had grown up believing all his life that he would become his father's political successor. Yet when he asked Ichirou about the matter, the older man said that it would be Koichi's younger brother in Tokyo that would be the one to succeed him. He ordered his eldest son to help his brother in Tokonosu.
Koichi, who had no knowledge of his half-brother's existance prior to his father's revelation, was outraged that his father had another child behind his mother's back. At this point, Koichi lost all respect for his parent and grew an intense hatred of him. However, knowing better than to disobey his father due to his polictical power, Koichi worked with his brother. In secret, he used their partnership to pinpoint his brother's weak spot; in order to gather enough materials to put an end to the entire Shido family.
At one point, Ichirou phoned his son and told him that an officer in the public safety department was investigating the "real" source of his funds. Koichi was told to hold back the officer's daughter, Rei, by a year in order to humiliate him and make him back off. The teacher had no grudge against Rei, but at the same time, he did not care if he treated her the same way as the other girls, since he never had any interest in women to begin with. Koichi flunked Rei without hesitation, hoping that it would bring him to ultimately achieve his final goal.
He appears to be a kind and caring teacher, but in reality he is sadistic and allowed the students he had deemed "weak" to be bullied. There were even instances where he himself would engage in bullying. When the zombie apocalypse occurred, Shido used his charm and charisma to convince the majority of the surviving students to join him. After having brainwashed them Shido reveals his true intentions by having his followers engaging in sexual intercourse. He does this because he wants them to repopulate the planet, this way, he will rule and be served by mindless followers.
Shido has shown in several instances that he is quite ruthless, being willing to do underhanded things, such as lying and manipulating, in order to get ahead. He has also shown that he is quite arrogant, believing himself and the students that follow him to be superior to everyone else.
It should be noted that Shido is a coward. He is terrified of "Them" and immediately flees whenever he sees them. He also cowers in fear when Hirano points a gun at him.
His behavior throughout the series can be officially classified under standard psychiatry/psychotherapy guidelines as extreme sociopathy and psychopathy. His flawed psyche most likely developed as a consequence of growing up in a broken household that was dominated by his abusive father. So abusive was his father that his mother commited suicide when Shido was only a young boy; this tragic event in his past must have strained his mentality and made him into a man without humanity.
It was implied by Hirano that Shido allowed bullies to beat him up as he watched with glee. It is assumed that the former's story is true because he pointed the nail gun at Shido without blinking or hesitating to punch a nail wound to his face. He has a desire to be a leader and some of the characters have theorized that he is trying to build a new religion with him on top as leader. Shido is an apparent coward in the face of extreme danger, as seen when being threatened by a grazing head shot with a nail gun on Hirano's part, but nonetheless, proves to be a manipulative, villainous and sadistic character. He is last seen by the main group when they abandon him on the bus, leaving him with only those willing to follow him.
He reappears once more and is seen watching over an orgy between his followers on the bus, in a sort of Charles Manson-ish brainwashing (he allows his followers to do as they will, calling it "free time"). He seems to have a spy inside the base of humans set up at the mansion, who, is relaying vital information about the morale of the people there. He is seen to keep his followers in line through intimidation, forcing them to follow him lest they incur his wrath or be left to fend for themselves. He sees the people that follow him as angels that will help usher in a new age, since they are youthful teenagers.
Shido then accuses one of the teens on board the bus of showing weakness, the boy who was showing concern for the welfare of his family. He convinces the rest of the group to eject the 'weak link' for the sake of the New World. Highlighting how far Shido has turned the minds of his followers, they callously throw the teen off the bus, going so far as to bid him farewell as he attempts to get back on the bus, only to fail and be subsequently attacked by a zombie. The teen is later seen in his undead form when the dead break into the estate.
Later on, his history and background with Rei was shown when his father was involved in Japanese politics and had Rei retained as a favor of getting back at his stepbrother. When threatened with a rifle mounted bayonet by Rei he seems scared witless until he realizes that no one would come to his aid before daring her to kill him. When she turns away and says that he is not even worth killing his fury is shown. He is seen being ordered to leave the estate with the students he brought but ends up crashing into the forklift when the EMP Blast temporarily stopped the bus, removing the concrete blocks which served as the barrier against the undead. He recovers consciousness in time to see the undead horde advancing through the gap in the barrier.
Shido manage to escape the bus wreckage, and he is later seen with Yuuki Miku and Tsunoda standing near the entrance of the evacuation camp at Shintoko Third Elementary School. He opens his arms wide, smiles, and asks his followers if they are ready.
- Fans of the series often compare Shido to Orochimaru from the Naruto series due to his somewhat snake-like appearance. He also bears an uncanny resemblance to Koshaku "Papillion" Chono of the Buso Renkin series.
- Koichi Shidou is also very similar to Terumi Yuuki.