Sou Hishigata (菱形窓, Hishigata Sō) is a 17-year-old boy who is Shinpei's childhood best friend. He is the eldest son of Hishigata Clinic, the only hospital on Hitogashima, and is studying to become a doctor. He has a strong sense of justice and regrets that he was present at the scene of Ushio's maritime accident but was unable to prevent it. He has confessed his feelings to Mio twice in the past and was rejected.
Sou is a young man with messy brown hair and brown eyes.
At Ushio's funeral, Sou hugs Shinpei and apologizes for not being able to help her. He offers Shinpei to punch him until he feels better, but Shinpei refuses. Sou explains that Ushio died while saving a kid, Shiori Kobayawaka, but the girl haven't spoken since then. That evening, Sou calls Shinpei and explains the rumours about his father performing an autopsy is false, but they did find strangler marks on Ushio's neck. However, given the situation and the police labelled it as an incident and only a few people know about the neck marks.
When Shinpei lives the day for a third time, Sou visits him at Kofune's Diner, where Shinpei tells him about Shadows. Mio suggest they go to Hiruko Shrine and speak with the priest, but Shinpei convinces them to go the next day during the Summer Festival, as being more crowded would be safer. Before leaving, Sou wants them to have a code word and Mio suggest Mesopotamian culture. Sou then leaves, wanting to check on something before tomorrow, but the next when Shinpei asks him, Sou states it was nothing. During the festival as Shinpei blames himself for not protecting Ushio, Sou tells him it wasn't his fault and he need to protect Mio now, but if he doesn't, Sou will protect her for him, making Shinpei realizes that Sou still has feelings for Mio. Sou explains that after the fireworks, the priest would be free and they can visit him then, but Shinpei sees something and leaves the group. When the fireworks end, Mio calls Shinpei and he joins them. After the ceremony, Sou decides to confess to Mio, but Mio explains she wants to be friends with him. Ushio then appears and comforts Sou, but everyone see through her disguise. Mio thinks she is a shadow and warns Shinpei to stay away from her, but Shinpei himself reveals he is a shadow.
Other shadows appear and they start killing people. A shadow that copied Sou, attempts to kill Mio, but Sou uses his body to protect her and Shadow Sou kills him, hitting him in the head with an axe.
Sou meets with Shinpei at Kofune's restaurant. Shinpei takes two umbrellas and they leave, which surprises Sou as its sunny outside. They go to the beach, where Sou wants to tell him something related to Ushio's death. Ushio then shows up behind him and guesses its the strangle marks, which makes it look like she was murdered. Sou confirms, but then sees Ushio and ends in a shock. After they tell him everything, Sou apologizes for not being able to save Ushio and wants she to punch him. Ushio then kicks him and tells him to get up as they rely on him. Shinpei gives her phone and after unlocking it, they find a video from the morning when Ushio died. Watching the video, they see Ushio and her Shadow Ushio on it. Ushio tells him that Shadow Ushio is on their side and continues to explain how she met her Shadow and formed a plan to save Shiori. When the video ends, Shadow Ushio regains her memories and abilities and decides to replay them the memory of how Ushio died. She transports Shinpei and Sou on the private beach on 21st. They witness how Shiori is being dragged underwater by her Shadow and Ushio and her Shadow trying to save her. They manage to take Shiori from her Shadow, but Shadow Shiori then strangles Ushio and captures Shadow Ushio. She attempted to fix the faulty Shadow, but Mio and Sou came and she pretended to be the real Shiori, while erasing the real one. While Mio carried Shiori, she grabs Shinpei's arm, surprising him, thinking that was just a memory and they couldn't interact. Shiori then turns to Heine, who states she found him and won't be letting him go. Shadow Ushio ends the replay and Shinpei sees a mark on his hand where Heine grabbed him. Shinpei decides they need to investigate the old clinic.
On their way to the old clinic, Sou reveals that when he was in fourth grade, he tried to sneak in the clinic as Tetsu Totsumura and his friends invited him for a test of courage. However, he wasn't able to get in as his father caught them. Since they owned the clinic, Sou didn't think much of it as it was possible for Seido to be there, but he finds it weird and wonders what his father was doing there. They go to the old clinic and while looking around, they find an old wood Hiruko statue. Shinpei notices a crack on the wall and asks Ushio to go, who then sees the wall is a secret door but it needs a key. Shinpei figures the Hiruko statue looks pregnant and Ushio scans it, and copies the key from inside. As they open the secret door, Ushio tells them she feels a shadow down below and has a bad feeling. Sou wonders what his father is doing as there is a secret door that can only be opened with Shadow's help. As they go down, they see its an old tunnel dug during the war. Shinpei asks Ushio to prioritize protecting herself and Sou to protect her as well. He explains that if he dies, he will loop, but if Ushio dies first, she may not loop back with him. Heading towards the shadow, they see a baby-like shadow, who appears to be deaf and blind, but manages to smell them. The shadow jumps on Sou, but Shinpei manages to pin it with his nailgun and Ushio kills it. Ushio explains that the first shadow she felt is still down the tunnel and they continue. They hear a gunshot and end in a natural cave, where they get surrounded by baby-like shadows. As they fight them back and try to escape, Shinpei notices that the shadows are only attacking Ushio. Once they manage to escape, he sees that Ushio isn't well and her wound from Mio haven't disappeared. Ushio then explains that she needs to copy the original to restore the lost data, but as original Ushio is now ashes, she can't heal the wound. Tokiko then appears, thanking them for bringing the defective product, Ushio. Shinpei shoots at her shadow to pin her, but Tokiko laughs and reveals she is human. Having no choice, Shinpei decides they to surrender, figuring that they won't hurt Ushio right away, he wonders if she will take them to her father. Tokiko tells Sou that she no longer needs to keep secrets from him, but its natural that he doesn't know anything, as he wasn't chosen by their father. As they follow her, she takes them to Hiruko, who Shinpei recognizes as Heine.
Tokiko explains that Hiruko like to be called Heine and look like her, who is the first person she copied. Recently she had troubles walking and needed a new body, so she took Shiori. She explains that her family have been serving the shadows for generations and they bring the dead to her for she to eat them. As they bring Ushio's body, Tokiko states Ushio's death was a surprise to her and never expected she to team up with her own shadow in order to save Shiori. Sou can't accept this and yells at Tokiko, asking her where their father is. Tokiko yells back that its because he is like this that she had to carry everything herself. She had to do all of this, because if Heine dies, their mother would die too. Their mother wasn't cured and their father asked Heine to turn her into a Shadow, so she can live. Soon, the shadow parent shows up and brings Mio as hostage. Tokiko is angry as they promised to not touch Mio. The shadow finds this foolish as all the humans on the island would be dying tomorrow at the festival. Tokiko wonders what he means as tomorrow Heine was supposed to have her final meal and heal her body, and then bring her mother back. The shadow shoots at Tokiko, but Sou gets in front and takes the bullet.
- The name Sou means "window" (窓).
- Sou's surname Hishigata means "Rhombus, diamond shape" (菱形).