Haruo Kawaguchi is the youngest of the four Kawaguchi siblings, which include his older brothers Fuyuo and Akio, and his older sister Natsuko. Growing up in Water Station 30, Haruo and his siblings had a formative and troubled relationship with Kanta Mizuno, the protagonist known as Desert Punk. During their childhood, Kanta convinced Haruo and the other Kawaguchi children to engage in petty theft and bullying. In one such incident, Haruo and his sister Natsuko were caught while attempting to steal from a shop. The relationship between the Kawaguchi family and Kanta turned violently sour after Kanta attempted to assault Natsuko; upon learning of this, Haruo joined his two older brothers in beating Kanta severely.

In his role within the story, Haruo typically appears alongside his two brothers, Fuyuo and Akio, as a supporting member of their sister Natsuko's unit. He is loyal to his family, frequently seen assisting Natsuko with her missions and expressing a desire to help her in any way needed. Despite this dedication, Haruo is portrayed as less capable and more prone to distress than his siblings. During combat situations, he is easily frightened, such as when he froze and raised his hands upon being confronted by an enemy soldier. He is also shown to be somewhat physically fragile or prone to complaint, once falling to the ground and citing a stomach ache when preparing to undertake a task for his sister. In one particular engagement, he lost his weapon, demonstrating a lack of composure in the field, and was described as dizzy while moving with the rest of his group.

No reliable direct information about Haruo Kawaguchi's appearance, specific combat techniques, or unique abilities could be found in the provided search results.