Mitsukou

"Who knows? Something important to me... Maybe?"

- Mitsuba to Kou.

Mitsukou is the slash ship between Mitsuba Sousuke and Minamoto Kou from the Toilet-bound Hanako-kun fandom.

Canon
Sousuke and Kou were former friends and classmates in junior high. They weren't best friends, per se, but due to sitting in name order, Sousuke sat in front of Kou. They seemed to get along well, and became friends fairly quickly. Sousuke ended up moving classes, however, and later died in an unspecified accident. Sousuke reappears as an apparition, though he has a brand new look. He roams the school entrance, and reaches out to those he remembers asking if they remember him, though he looks more like a silhouette. When Sousuke reaches out to Kou, Kou grabs his hand and knocks him back into a locker, revealing Sousuke's true form. Kou didn't recognize him, but he did recognize Sousuke's male school uniform.

At the time, Kou had made up his mind to only exorcise spirits that cause harm to others, so he had spared Sousuke and decided to help him get rid of his regrets instead. Initially, the two did not get along. Sousuke constantly lashed rude nicknames at Kou, his favorite being "Lame-ass traffic-safety-earring boy" in which he'd refer to Kou on multiple occasions. He'd also assume Kou was trying to "do pervy things to him," resulting in Kou getting called a pervert often. In return, Kou thought Sousuke was rude and obnoxious, though he helped Sousuke with his regrets anyway.

At first, Sousuke refused to answer any questions that Kou had, leading to Kou threatening him to get answers. Eventually, Kou learned that there was a picture Sousuke wanted to take when he was alive, so Kou decides to help him take that picture. That was when Kou's friend, Yokoo stops by to say hello to Kou. Yokoo notices the camera Kou is holding, and said it belonged to Mitsuba, and gave some insight on who he was. This helped Kou remember Sousuke, and he felt bad for not recognizing him at first. Sousuke then tells Kou "I reached out to everyone who I thought was my friend, but nobody recognized me.. only, I remembered them."

It began to rain, so the two decided to head inside together, taking the stairs to the top floor. On the stairs, Sousuke reveals that he was bullied in elementary for being too "girly" or "cocky," so he decided to build a new persona in middle school. He was nice to everyone, but nobody noticed him. He wasn't bullied, but he didn't make any real friends. When he died, nobody seemed to notice or care. Kou then decides to tell Sousuke that he's "cocky, girly, selfish, sarcastic, has an annoying voice, obsessed with his camera, goes emo sometimes, and a faker" and that he needs to be himself to make friends.

When Sousuke asks if they could have been friends if he was alive, Kou tells him that it doesn't matter if he were alive or dead, because they could still be friends. Sousuke then takes a picture of Kou, and when asked what he was shooting, Sousuke smiles brightly and responds with "Take a guess! It's something important to me.. Maybe?" before changing the subject and rushing Kou to go inside. Before they enter the building, however, Sousuke presses his head up against Kou's back and says "The photo will be ready tomorrow, just stay with me until then, okay?" to which Kou agrees.

When Tsukasa finds out about their friendship, he was confused to see Sousuke satisfied with just one friend, and revealed that Sousuke had wished to make friends, and to be with those friends. Tsukasa had granted his wish, but in return changed Sousuke's rumors, unwillingly changing Sousuke into the "Broken-Necked Boy" - a spirit that would break one's neck if they didn't remember his name. Tsukasa attempted to get Sousuke to kill Kou, but Hanako had saved Kou before that could happen. In a less distorted form, Sousuke was left lifeless, and unable to go back to normal. This deeply affects Kou, who's left with nothing but Sousuke's camera as a memory. He swore he'd never forgive Tsukasa for what he did to Sousuke.

In the Hell Of Mirrors arc, Sousuke appears again, though he has no memories of Kou. This "new Mitsuba" was made by Tsukasa from spirits that have nearly disappeared and a bit of Sousuke's soul. This Sousuke was a new person with no memories, and the Sousuke that was once human is gone forever, however, Kou wasn't discouraged. Though it upset him when Sousuke asked "Who the hell are you?", Kou was determined to rebuild Sousuke's memories again.

As time goes on for the two, they grow closer and become great friends. Kou tells Mitsuba that he should die too so they can be together forever, but Sousuke obviously refuses to let him do that. Despite the two being on opposing sides after Mitsuba joins Tsukasa, they still remain close friends. Despite this, Kou is still desperate to "get Mitsuba back" from Tsukasa.

Fanon
On AO3 Mitsukou is the 2nd most written ship in the Toilet-bound Hanako-kun tag and the 1st most written ship for both characters.