|Name Kanji||結城 Yuuki|
|Anime Debut||Episode 1|
|Voice Actor||Mike McFarland|
Father of Natsuno. He works as a carpenter.
He is calm, collected and very protective of his family, especially his son, Natsuno. He and his common-law wife reject social norms and have never gotten married.
Yuuki hates superstitions and religions, and therefore can't grasp the idea of vampires existing. He has been teaching his son not to believe in such things. In some way, he is indirectly guilty for the death of his own son.
He had his first child, Natsuno Koide, with Azusa Koide while they were in college. They questioned the value of marriage and never finalized anything on the family register. Their son has Azusa's surname for this reason.
On August 11, he, along with Tomio Ookawa, Atsushi Ookawa, Mutou, and other villagers, search the mountain for Megumi Shimizu. While walking with Mutou, he spots the police man Takami in front of Kanemasa's. Takami says that Seishin Muroi and Toshio Ozaki said that Megumi Shimizu might have came up there, so he wanted to ask them if they seen her, but he got nervous dealing with people from the city, so he asks Yuuki to ask them, since he's from the city and he agrees.
Using a PA system, he says that they're looking for a high school student and asks if he could ask them some questions. Tatsumi asks them to wait a moment and then opens the door. He says that nobody in the house has seen her but offers to help search for her, because the master of the house said that everyone should help one another. He says that his name is Tatsumi and he's a servant and asks how he can help.
After hearing other villagers gossiping that they might have something to do with it, Mutou tells Tatsumi not to pay them any mind.
After the disappearance of his wife and the death of his son, he goes mentally insane, getting worse when Natsuno comes back to their house as a jinrou to live with him again. He spends the days cooking food for his son since Natsuno avoids drinking human blood. Then later when Tatsumi arrives, he stabs him, maniacally, however Tatsumi recovers from it, and it seems Yuuki doesn' t care to let Tatsumi inside the house.
Tatsumi goes to his house. Yuuki stabs him with a knife as he rings the doorbell. He is overjoyed to have killed one of the undead, but the wound doesn't hurt him and Tatsumi asks to come in. Yuuki is happy to oblige.
At the end he somehow survived the fire and regains his sanity where he and Kanami went into the bus and then he sees Kaori and Akira smiling. It is implied he would look after them afterwards.
In the manga when Natsuno is being dangled over the pit by Tatsumi, Yuuki approaches them from behind and shoots the latter, maniacally cheering to himself for punishing the person who is bullying his kid. This distracts Tatsumi long enough for Natsuno to attach a handcuff to Tatsumi and himself before dragging the both of them down into the pit to their demise. Yuuki waves goodbye to his son while Natsuno tells his father to take care as the latter falls out of view.
- He is the first person to become insane after he lost his partner and son to the Shiki and the only one to recover.