Kazuichi Souda is a student of Class 77-B at Hope's Peak Academy, where he holds the title of the Ultimate Mechanic. His life prior to attending the academy was marked by economic hardship and familial strife. His family owned a financially struggling bicycle shop, which rarely turned a profit despite the hard work involved, leaving them with very little money. This environment forced Kazuichi to practice and refine his mechanical skills from a young age, taking things apart and rebuilding them. In addition to poverty, his home life was abusive; his father was an alcoholic who would beat him when he missed school or when bike sales were poor. He was also a victim of bullying for his unkempt appearance which was associated with poverty, leading him to seek solace in mechanics workshops.

To cope with his difficult childhood and past betrayals, including a middle school incident where a friend falsely accused him of cheating which led to his social ostracization, Kazuichi constructed a new persona for himself. Before entering Hope's Peak, he drastically altered his appearance to project a "tough punk" image as a defensive facade. He dyed his hair pink, discarded his glasses for hot pink contact lenses, and sharpened his teeth, creating an intimidating look that clashes heavily with his true personality. This persona, however, is constantly betrayed by his genuine nature.

Kazuichi possesses a highly emotional and compulsive personality that is often at odds with his flashy appearance. Despite his best efforts to appear cool and carefree, he is fundamentally cowardly, very jumpy, and easily frightened, often screaming in panic over minor incidents. He is a very bad liar, a trait that leads him to blurt out his thoughts without a verbal filter and to quickly contradict himself. This sensitivity and timidity frequently make him the target of teasing and jokes from his classmates, such as Hiyoko Saionji, and even Monokuma. While he is often seen carrying a wrench and becomes anxious when he cannot tinker with something, Kazuichi's primary passion is for all things mechanical, particularly fast machines like rockets and motorcycles Ironically, despite this love of speed, he suffers from terrible motion sickness.

His most defining and problematic trait is his obsessive, unhealthy fixation on his classmate, Sonia Nevermind. He is infatuated with her primarily for her appearance, as he claims he has always dreamed of having a beautiful blonde girlfriend. This obsession manifests as stalker-like behavior, including schemes to "coincidentally" encounter her, and intense jealousy towards anyone who attracts her attention, particularly Gundham Tanaka. In a notable incident, while under the influence of an aphrodisiac, Kazuichi attempted to take advantage of a defenseless Sonia, an action that underscores the toxic nature of his crush. He also tends to dismiss her personal interests, such as her fascination with the occult, as he prefers to keep an idealized version of her in his mind.

Within the story of the Despair Arc, Kazuichi plays the role of a supporting character in Class 77-B. He often acts as a grounding presence, serving as a straight man to his many eccentric classmates. His key relationships include his friendship with Hajime Hinata, whom he eventually comes to trust as a "soul friend" despite his general distrust of others. His Ultimate Mechanic talent makes him an invaluable asset to the group. He is responsible for designing and building numerous devices, such as constructing a monitor for the class to play video games together, which serves as a bonding experience, and fixing the critical elevator system that operates between two buildings. He also demonstrates a caring, if sometimes misguided, side by building a small robotic toy named Minimaru from the spare parts of Nekomaru Nidai to help comfort their friend Akane Owari after Nekomaru's death.

His character development in this prequel arc is largely one of tragic descent. Kazuichi, alongside the rest of Class 77-B, is a target of Junko Enoshima's scheme to plunge the world into despair. His transformation into a Remnant of Despair is not a willing choice but the result of brainwashing. Junko forces the entire class to witness the execution of their beloved classmate, Chiaki Nanami, through brainwashing footage. This act overloads them with despair, turning them into Ultimate Despair. Following this, Kazuichi becomes a willing participant in global atrocities, showing a newfound ruthlessness such as the indiscriminate killing of civilians, while still retaining his obsessive fixation on Sonia. Ultimately, after the events of the Killing School Trip and the Neo World Program, Kazuichi survives and his memories are restored. Along with the rest of his class, he accepts responsibility for his actions as a Remnant of Despair, voluntarily agreeing to live in exile on Jabberwock Island as a form of penance. There, he returns to his mechanical work, helping to maintain the island's infrastructure and even pursuing his dream of building a spaceship.