Being quite literally invaded by an alien does things to a person. Not only has Shinichi's life experiences as of late been shaped by that abrupt forced factor into his life, but his personality has been affected by it as well.

Growing up, he was perhaps the definition of A Very Good Boy. In fact, his parents frequently remarked that he was too well-behaved (his only real dark deed being to accidentally cause his mother to burn herself with hot oil when he was really young, leaving a large scar on her arm that would always remain a source of guilt in Shinichi's gut) and worried over their son often, but Shinichi was happy being a well-mannered, boring, high schooler. He had always been on the shy and soft-spoken side, never one to demand attention or even seek it out in small doses, preferring his status as a wallflower. Social awkwardness was part of the package as well, but he did have a close-knit little group of friends (100% girls; most of the boys his age and their interests were far too rough for his gentle demeanour). His sensitivity showed itself in other ways as well: he always had a weak constitution for things like horror, violence, confrontation, and insects.

However, it is Shinichi's altruism that has truly defined him as a person. Despite being someone whom others may easily peg as cowardly or "weak", if a situation involved someone else being hurt or in trouble, Shinichi would interfere without a second thought as to his own safety. This has led him to literally step into the line of fire, whether it be protecting a classmate being targeted by bullies and ending up beaten up himself, or moving into traffic to protect a child. He recognises that such acts can be foolhardy, but at the same time his protective nature and strong sense of altruism are not traits he's ever thought to challenge.

Because Shinichi is absolutely led by his heart, and that's what has always felt natural to him.

So then that aforementioned alien invasion happened, and literally every aspect of Shinichi's personality was challenged. Suddenly he was host to something not only outright terrifying in appearance, but also capable of extreme violence -- violence that Shinichi found himself unwillingly exposed to, as other parasites became hostily drawn to him and Migi, forcing the pair to have to destroy them first. For Shinichi, who had never hurt a fly and never wanted to, bloodying his hands was something he couldn't come back from. Not to mention the nightmares involved with being hunted by monstrous parasitic organisms hiding in the brains of humans. The paranoia from that, coupled with having to hide this dark secret from everyone around him, led his psychological state to be quite tested.

There was also the fact that he was now sharing his life and body with a cold, "insect-like" being without emotions. Something the complete opposite from himself, who was concerned solely with its self-preservation. This challenged many of Shinichi's values and morals, he and Migi frequently clashing over their ways of thinking and behaviours. Suddenly terms like "right" and "wrong", "justice" and "mercy", and "kind" and "cruel" began to fall on a more grey spectrum for the human boy. He initially fought it hard, viewing Migi just as much an "alien" to him as the parasite did Shinichi. They only used each other as needed, Migi relying on Shinichi's body for literal substance and Shinichi relying on Migi to protect him (and therefore his loved ones) from the other parasites.

Inevitably, as time wore on and they were forced to help each other's mutual goal of surviving day-to-day by combining each other's strengths and covering each other's weaknesses, he began to feel a sense of unease that he was starting to be able to understand Migi in ways his past self would have balked at. The most disturbing part of this, however, were not the ways he consciously began bonding with Migi, but the ways he subconsciously was growing to. The moments were small and insidious -- for example, Shinichi no longer experiencing some of his usual fears, things that Migi would perceive as irrational. More extreme examples included Shinichi literally threatening to eat a group of teenage boys who were hurting a cat (not to worry, Shinichi saved the little dude). The boys threw rocks at Shinichi, causing Migi to swoop in and catch them, and threaten to... eat the perpetrators, speaking through or perhaps mentally fused with, his host.

So a new fear began to blossom in Shinichi: that he would develop some sort of appetite for humans, and lose his precious humanity to Migi. After all, their situation held a great deal of unknowns -- even Migi didn't know what might happen in their odd case, where a parasite and its host retained their individual mindsets when they were originally supposed to fuse. It was a sort of catch-22: they were starting to be able to work together more efficiently as one unit...but to sometimes literally merge into one mind, as well.

In a nutshell, Shinichi Izumi is the result of someone who fell victim to a very bizarre experience by chance. None of his character traits have really changed -- he's still the same polite, mild-mannered, sweet, and altruistic person that he was before. But these traits have certainly been challenged and have been slowly, subtly evolving to where sometimes he's not sure if those things are really him anymore, or if he's having to grasp onto them instead. There's also the fact that carrying the weight of everything he has gone through has taken some of the "innocence" away from him. He may seem almost comically awkward and out-of-place on the surface, but beneath that there is something darker and heavier. He's been exposed to things that he can't return from, and having to emotionally and physically withdraw from his loved ones in order to hide Migi has taken an extremely heavy toll on him, as well.

Despite the moments of merging with Migi, there does remain a rather clear divide between them most of the time. If Shinichi is the emotional heart, Migi is the logical head. But perhaps the most accurate representation of them could be described when a young girl who had psychic abilities looked into Shinichi's eyes and was terrified to detect the image of a large, alien eyeball slowly opening itself wider and wider. Migi may not have successfully taken over Shinichi's brain, but deep down, the alien is inside the human.

Personality-wise, Migi is highly curious and outright inquisitive to the behaviour and life of other species. He will not hesitate to act on his own to satisfy this scientific curiosity, often putting Shinichi in the most awkward of situations. He may seem approachable: being almost childlike in his curiosity, speaking politely and formally, and even hosting a sense of humour that pops up from time to time. Yet underlying this is a rather frightening side to his personality in that he lacks human emotions and sees things from a purely tactical perspective; his purpose is to keep himself alive, and that means keeping Shinichi alive -- at any costs. He possesses intelligence beyond that of human capacity and is capable of acquiring information in an incredibly short amount of time. He is constantly adjusting to life, learning as much as he possibly can not only about the human world but also his own existence, and spending long nights awake reading books or doing research online.