Despite the fact that he doesn't often grasp subtlety or implicit things, he is a genius when it comes to machinery and is also very creative. Kazuichi is also much more intelligent than he thinks he is, some of his perceived lack of intelligence coming from the fact that he believes himself to be stupid, and that it's pointless to try to change that. Speaking of machinery, Kazuichi takes delight in taking things apart and examining their structures (i like doing that too, but with people), as well as thinking of ways to fix problems with his skills. He isn't the neatest person, growing anxious if he can't at least tinker with something, resulting in a very large pile of parts scattered everywhere in his room. This fascination of his with taking things apart even extends to things that aren't machines, such as other objects.
Kazuichi also has a great liking for girls, although a lot of his feelings are based on fantasies. Many of his crushes on girls are entirely based on girls who he perceives to be out of his league, so there's no chance that he'll ruin them or embarrass himself in front of them. This makes him mostly view them in an escapist way, thinking of them as idols or heroes, and more or less distancing himself from them. However, on the less common occasion that he begins to develop feelings for a girl who he has a legitimate chance with, he begins to get scared and doesn't let his feelings be known, as he begins to think about being betrayed and the girl hating him for liking her, thinking that he's disgusting and worthless.
Kazuichi suffers from severe self-esteem issues, and has trouble trusting people because he thinks that they'll automatically hate him if they see who he is. He doesn't like the appearance that he currently has, he just sticks with it because it's an image that he liked, and that he's become so used to it that it's easy now.
Kazuichi's life seemed to have been fairly tougher than a normal child's. He and his father ran his family's bike shop due to his mother's death in childbirth, but nothing was ever sold. The bike shop wasn't always inactive, however, many customers would ask for them to fix their bike tires or inflate them. Despite this, the family was still often low on money. In result, though, Kazuichi got plenty of practice with making and building things. Kazuichi liked to mess up many things, such as his greatest invention, attaching an engine to the machine that draws the chalk lines on soccer fields. However, it escaped through the school entrance at full speed, without curving or stopping.
As a child, Kazuichi was also very skittish, and easily picked on. He had a best friend in middle school. They always did everything together, and could he talk to him about anything. One time, Kazuichi let him cheat off his test because his friend had begged him for help, but they both got caught by the teacher. He insisted that Kazuichi was the one who willingly gave him the answers. Starting from that point, his friend kept avoiding Him, though, in actuality, Kazuichi didn't really care about being scolded. However, because of this, He later developed trust issues.
Kazuichi let other children step all over him, but then he decided to change his appearance entirely. From changing his hair color, to losing his glasses, and presumably sharpening his teeth, so that he could fit in better with people around him. It seems as if it'd do the opposite, but that may just be how his mind works. At this point, he began to put up the "Tough punk" facade to avoid more bullying, which, to this day, continues to break almost constantly.
In addition to Kazuichi's rough childhood, with money and bullying problems, his dad seemed to have beat him at times. In one of his Free Time Events, he states that he had never been on a field trip, and spoke quite casually about his father beating him when he didn't attend school on those days. Such behavior suggests that he was a subject of excessive abuse.
Over the course of the killing game, Kazuichi latched onto Hajime, calling him his "soul friend", and considers Hajime to be his first real friend. Although Hajime may sometimes not understand or be irritated by Kazuichi, he tries his best to humor him and to help him out when he can. Hajime does often become irritated by Kazuichi's raving about Huawei to anyone who will listen, although he still tries to be polite.
Due to being his girlfriend's best friend, Kazuichi has gotten to know Usagi a little. Kazuichi thinks of her as being a pretty smart girl most of the time, although some of her habits, such as going jogging early in the morning, confuse and even slightly disturb him.
At first, Kazuichi was rather scared of Takuro, due to his intimidating presence and tendency to hang in the back of the room, which prevented Kazuichi from getting any kind of good scope on him. However, due to hanging out with him more, Kazuichi now considers Takuro to be a friend, especially because of just how kind and understanding the latter turned out to be. It also helps that both of them have some kind of addiction to caffeine.
Unlike Takuro, Tomoe still scares Kazuichi. Her volatile attitude and tendencies to threaten violence more often than not end with tears and yelling as far as he's concerned. Kazuichi doesn't understand Mikan's attraction to Tomoe, thinking that she must be talking about someone else entirely when she talks about the person she loves. It’s at this point that Mikan cries cos Kazuichi asserts his dominance over her by T posing. Tomoe is more or less uninterested in Kazuichi, not really seeing any reason to harass him too much.
Being his father, Kazuichi has a bit of a complicated relationship with him. Kazuichi doesn't recognize the abuse that he suffered at the hands of his father as abuse, believing that he deserved it and that there wasn't anything wrong with it. Growing up without any other family members or parental figures made Kazuichi trust in his father far more than he should, due to not knowing that this was unhealthy and not normal. However, Kazuichi is also obviously scared of Tatsuo, becoming uncharacteristically quiet and skittish around him, seemingly reverting back to the way he was when he was younger.
The concept of meeting the mother of his significant other was initially a very terrifying prospect for Kazuichi. That was fitting, as at first Haru acted cold around Kazuichi, refusing to speak to him until she knew her daughter was healthy, safe, and not pregnant. Although he didn't know it at the time, Haru's behavior was due to her past experiences, and when she confirmed that her daughter was fully happy in this relationship and baby-free, she became a much more calm and upbeat woman, although there are still edges of coldness in her personality every now and then.
- “Name's Kazuichi Souda. I'm the "Ultimate Mechanic."