Mio Kofune (小舟澪, Kofune Mio) is Ushio's younger sister and a first-year high school student. She is good at sports, belongs to the swimming club, and has tanned skin. Despite the death of her older sister, she has a healthy side, such as her cheerful attitude toward Shinpei when he returns home.
Mio is a young girl with medium long black hair and blue eyes. She also has a tanned skin.
After two years away, Shinpei Ajiro returns to Hitogashima to attend his childhood friend Ushio's funeral. Stepping on the island, Mio welcomes him and they head towards the funeral while chatting about various subjects.
The next day, Shinpei and Mio work at Kofune's dinner. The police officer, Tetsu comes and explains that the whole Kobayakawa family had vanished. Hearing that, Mio runs away and Shinpei follows her. After catching up to her, Mio explains that Shiori said she saw a girl looking like her about a week before the incident with Ushio. An old hunter, Ginjiro Nezu, tells them that's a shadow and those afflicted by shadow sickness, see their shadow and then get killed by it. The shadow then acts as the killed person and kills the remaining family. It used to be quite common sickness before the war and those afflicted went to Hito shrine for cleansing. Mio then states she saw Ushio's shadow along with Ushio, three days before Ushio died. Shinpei then decide they go to the shrine to learn more about the shadows. However, the shrine is closed at the moment and Mio sees Shiori in the forest and they run after her. They are unable to find Shiori, but hear a gunshot and following the sound, they see the woman with glasses bleeding from a gunshot. Mio heads to get help and the woman tries to tell Shinpei something, but get shot in the head. Shinpei then sees Mio holding a gun and holding the Mio that was with him as hostage. The other Mio then shoots Mio and Shinpei.
On his third time stepping on Hitogashima, Shinpei manages to stop Mio from falling in the water, but he ends up falling in it and she jumps to save him. That evening, he records the Shadow Mio outside of their house and in the morning shows Mio the footage, explaining its a Shadow that came to kill her. As he is aware of Ushio seeing her shadow, Mio guesses that Ushio told him that and Mio then realizes that Ushio was likely killed by her Shadow. Mio tells him that Shiori also saw her shadow, but Shinpei decides to go alone.
Later, Shinpei returns to Kofune's Diner, and moments later Tetsu comes, explaining that Kobayakawa family is gone. As Sou comes, Shinpei tells them about everything and Mio suggest they visit the priest as he may know something. The group then decide to go during the Summer Festival tomorrow. The next day, they go to the festival, but before meeting the priest, Shinpei sees something and runs, leaving them. When the fireworks stop, Mio calls Shinpei, but hears Ushio. She is shocked, but Shinpei explains she is imagining it. Mio then tells him to come to the shrine and soon Shinpei returns to them. Sou uses the chance and confesses to Mio, but as Mio loves Shinpei, she tells Sou she just wants to be friends with him. Ushio then appears and comforts Sou, but everyone see through her disguise. Mio doesn't believe that's Ushio and calls her a monster, warning Shinpei to stay away from her, but Shinpei refuses and grabs Ushio, wondering what she was thinking for trying to switch places with someone who had public funeral. The group then realizes that Shinpei is a shadow. Other shadows come and they start killing people. Shadow Sou attempts to kill Sou, but Sou uses his body to protect Mio. The real Shinpei then arrives and they attempt to reach each other, but she ends up being consumed by the shadow.
On the fourth time, at the funeral, Shinpei asks Mio if she wore white panties yesterday. He recalls that Shadow Mio had white panties, but today Mio is with different color. Mio finds it weird that he would ask her something like that after they haven't seen each other for so long, but after he tells her its important for him to know, she confirms it.
At home, Mio decided to cook to make Shinpei feel better. As he finds her doing it, he decides to take over and tells her to go wash herself. When Mio returns, she hears his stomach rumble and relaxes as she guesses that means she is fine. Shinpei goes to eat in his room and takes two plates. Later, Mio comes to take the plates, but is surprised he ate from two spoons and used two glasses, guessing that Ushio visited and explaining that in Hitogashima, they believe that the soul of the dead returns to their home and protects the family as their deity. She ends up spending the night talking to Tokiko on the phone and in the morning gives the shell necklace to Shinpei.
Mio and Tokiko are hiding from the rain. Tokiko wonders if Mio will be going to the festival tomorrow, but Mio explains she doesn't have a yukata. Tokiko states she will gladly lend her one. Tokiko then wants a favor from Mio and asks her to stay with her tomorrow. A man then shows up, but seeing he isn't local, Tokiko wonders who he is. The man introduces himself as Miura from Wakayama's northern station and asks them if they knew the Kobayakawas.
Mio tries to call Shinpei and Sou, but they don't pick their phones. She sees Shiori, who whispers that Shinpei is that way. Heading to the corner at the street, Mio encounters Tetsu Totsumura and they see Shiori pointing at a manhole, which is slightly opened. Tetsu doesn't want to go there as he will stink and Erika won't like him, but Mio states that if he finds the missing girl, he will be a hero and girls will be all over him. That convinces Tetsu, who then leads them into the sewers. Following the sewers, Tetsu starts to complain, but Mio tells him he will tell Erika how brave he is, so he decides to continue. They reach a dead end, but Shiori flashes and eats the iron bars, allowing them to proceed. She is then captured by the shadow parent, who brings her to Heine and uses her as a hostage to make the others obey. When the shadow no longer needs her, he crushes her skull and kills her in front of Shinpei.
- The name Mio means "water route, shipping channel" (澪).
- Mio's surname Kofune means "boat, small craft" (小舟).