Because Seiichi was almost killed by his mother in this area, this might be the manifestation/hallucination of the trauma caused by his mother. Shigeru was made into an outlet for Seiichi's mother's rage just like Seiichi was before and this could be a way for Seiichi to finally sever his connection to his mom. I don't know where this is going. Maybe he will push "Shigeru" off signaling his path in life will mirror his mother, or that he's not going to be the victim anymore. Or maybe he has to save "Shigeru" to deal with his repressed trauma.