Shinichi is a thoughtful and nice person who is considered a bit of a wimp. But that changes after the incident in which his mother was killed and taken over by a parasite, resulting in his drastic change in both his mentality and physiology.
As the series progresses, Shinichi becomes somewhat distant and unsympathetic, but also gains resilience. His bond with Migi becomes stronger and he occasionally begins to think like a parasite, as how he described the dead dog's body as 'a lump of dog-shaped meat'. He also loses the ability to empathize with others and even losing his ability to cry. This was shown when he came into his father's room at the hospital, unable to speak as his grieve was too powerful and deep, but he could shed no tears. However, he also gains significant resolve and resilience, and is willing to risk his own life to protect his friends, family, and the public from harm. However due to recent events to the death of Reiko Tamura, he currently has reverted back into an emotional state, accepting his mother's demise and showing emotion once more.
A young Shinichi comes home from school one afternoon, and asks his mother for an aluminum plate for use in an assignment. Though she tells him to wait for her to get it for him, he tries to reach the high shelf where the plates are stored himself, falling in the process. When he falls he hits the stove, causing a pot of boiling oil to spill over, his mother quickly grabs the pot, saving Shinichi from harm, but getting badly burned in the process. Shinichi is often filled with guilt whenever he catches a glimpse of the burn mark on his mother's forearm.
Years later, Shinichi is seen looking at the results of his high school entrance exams. He is soon approached by a young Satomi, and after a brief misunderstanding, they learn they will be attending the same school. They express their excitement, looking forward to meeting again in April.