Smile and Sister Theories
  • There is speculation that Smile and Sister are related to Sabitsuki in some way, either by blood or relation in that they possibly shared the same illness.
  • Another theory is that Smile was one of her friends who she might have grown up with. Sabitsuki trusted Smile who betrayed her trust as they grew up. The fact that he's surrounded by school children could indicate that he became popular and perhaps eventually snubbed Sabitsuki altogether.
    • Alternatively, after he had grown popular, he and his followers, for whatever reason, decided to beat her up and he dealt the last blows before leaving her for dead, possibly driving insane and further complicating her illness, which had her hospitalized.
  • Smile is very protective about Sister as pulling out your pipe will lead to him pulling out his weapon as if protecting her as she hides behind him. Perhaps his protectiveness of his sister lead to him not being friends with Sabitsuki anymore because he didn't want her hanging around his sister with Sabitsuki's disease getting progressively worse.
  • A possible theory may be that Sabitsuki and Smile have done something wrong together. This could possibly explain the "Black Hood" and "Iron Pipe" effects. Smile also, of course, smiles when you hit him, possibly reminiscing on the act they performed. This could also explain why the Kaibutsu are so bloody and explain why Smile is in one of the school rooms alone with his hammer, as all the children flee from him. When he fights back during the corrupted school event, it could be an act of betrayal, for many unknown reasons. Sister seems to be afraid of Sabitsuki, possibly explaining this further.
  • Sabitsuki and Smile may have been part of a gang dedicated to keeping the diseased from harming the healthy. The plus (+) designs over the eyes may have been a form of uniform. Smiling and giggling may actually be an early symptom of the disease progressing in Smile, or the fact that he may have been driven insane fighting the diseased and adapted their mannerisms. The corrupted school event may be a memory of how Smile was driven to abandon Sabitsuki and betray her due to the disease affecting her too far. She also didn't fit in with the diseased due to actively fighting against them in the past, which would explain why so many are her enemies yet have no apparent problems with each other, since they often appear in groups. Alternatively, Smile may have been protecting Sabitsuki by knocking her out with the pipe before her pursuers could do any real damage to her.
  • There is a theory, linking to the theory concerning their possible friendship, regarding the marks around his eyes that state at some point, Sabitsuki was living with a dormant split personality named Rust, suppressing her while she was happy being friends with Smile and turned violent when Rust took over, slashing at Smile's eyes with a sharp object and alienating her from him.