Shinichi Izumi

Initially, Shinichi is a thoughtful and nice person who cares a great deal for those around him. However, he is also considered a bit of a pushover and a wimp, due to his awkwardness and paralyzing fear of bugs. All of that starts to change after being formally introduced to and acquainted with Migi, though he is understandably horrified at first. Shinichi then becomes more aware of his surroundings and excels in sports like basketball. He also overcomes his fear of bugs, due to Parasites being much scarier, much to the confusion of his mother. When Migi proposes that they work together by "cooperating and coexisting", Shinichi initially shows great reluctance, scared of both working with an alien symbiote and the thought of killing in order to survive. He considers human life precious and is disgusted by the Parasites killing humans senselessly, though Migi speculates that this is because it's his species that's being eaten and points out humans eat many different lifeforms in order to survive, and that, in a way, that makes them even worse than Parasites. Nevertheless, the two are shown to be able to cooperate in a crisis and work well together against hostile Parasites. After Nobuko Izumi's (Shinichi's mother) death and his subsequent rebuild by Migi, Shinichi gains significant resolve and resilience and is willing to risk his own life to protect his friends, family, and the public from harm. He becomes very calm and cool-headed, taking horrific events in stride without so much as flinching, a sharp contrast to his former awkward, timid self. His father even asks him outright if he's made of steel. However, he is not immune to anger, as he also shows a very vengeful side when he attempts to track down and kill the Parasite who killed his mother and took over her body, saying that the thought of such a being walking around freely makes his blood boil. However, as a side-effect of fusing with Migi, Shinichi becomes somewhat distant and unsympathetic. His bond with Migi becomes stronger and he occasionally begins to think like a parasite, as how he described a dead dog's body as 'a lump of dog-shaped meat' (Migi even notes that that sounds exactly like something he would say). This change in his behavior starts to worry those around him, especially Satomi Murano, who often wonders if it's really, he or someone else entirely. This is made worse by the fact that Migi often threatens to kill anyone who finds out about him, and so he is unable to tell people the truth about what has happened to him. Shinichi also gradually loses the ability to empathise with others and even his ability to cry. This is shown when he comes into his father's room at the hospital, speechless with grief, but unable to shed a single tear. It is also shown that he is still unable to cry despite feeling responsible for the death of Kana Kimishima and her subsequent funeral, much to the disgust of Mitsuo who speculates that he doesn't even care that she's dead. It is after this particular incident that Shinichi seriously starts to question exactly what he's turning into. However, after the death of Reiko Tamura, Shinichi reverts back to his former emotional state where he finally accepts the death of his mother and Kana Kimishima that he has regained the ability to cry again. Shinichi's opinion of Parasites changes a lot over the course of the series. At first, he considers them to be monsters, due to their being responsible for the 'Mincemeat Murders', and vehemently dismisses Migi's claim that humans are worse, though he begins to reconsider after rescuing a dog from a group of thugs. His belief that Parasites are monsters only strengthens after the deaths of his mother and Kana, believing that they must be wiped out and being frustrated by Migi's refusal to co-operate. However, after being saved by Reiko and encountering the psychotic Uragami, he admits to Migi that he doesn't know what's what anymore and starts to wonder who the real monster is. He ends up concluding that they are only trying to survive like him, and that he can only protect the small family he has around himself.

Shinichi is of average height with a small build. In the manga he has short black hair and brown eyes and is mentioned to have a somewhat feminine facial structure. In the anime, his hair and eyes are brown, and he initially wears large square-rimmed glasses. left|thumb|Shinichi's uniform comparison He usually wears his school uniform, consisting of a white shirt and a blue blazer, while in the manga he wears a Gakuran, an all-black, old-style Japanese school uniform, with a white shirt underneath. He also tends to wear T-shirts with strangely appropriate words written on them. For example, when he first meets Migi, he has a T-shirt that says, "Misery Confusion". This repeats throughout the manga. After Chapter 12 of the manga, or Episode 5 of the anime, Izumi is left with a large scar on the center of his chest and back. He becomes more muscular, and in the anime, he removes his glasses, since he no longer needs them. After Chapter 17 of the manga, or Episode 7 of the anime, Shinichi begins spiking his fringe upwards. He claims to like the new look because it "looks more balanced" - symbolizing that he is no longer merely a human, and also referring to his newfound calmness and mental fortitude. After his final confrontation with Gotou, he loses his spiky fringe and gains a new smaller scar over his right eye. He briefly wears a curtained hairstyle in Chapter 63, just before Migi goes dormant. Sometime during the one-year Timeskip, Shinichi reverts back to his original hairstyle, albeit more gelled.