Helmeppo is the son of the former Marine Captain Morgan, and he is introduced as a spoiled, cowardly young man who uses his father's authority to terrorize the citizens of Shells Town. He lives a life of luxury and privilege, believing that his father's power makes him untouchable and above the law. He is known for his distinctive appearance, initially having a lanky frame and a peculiar, sideways oval-shaped blonde hairstyle.

His personality at the start is defined by arrogance and cruelty, hiding behind his father's fearsome reputation. He threatens to have anyone who disagrees with him arrested or killed, and shows little regard for the lives of others. An early example of his pettiness is when he has the bounty hunter Roronoa Zoro arrested for killing his pet wolf, Soro, which had been attacking a child. He then breaks a promise to release Zoro after a month of starvation, revealing his dishonorable nature. Despite his cruelty, he is shown to be less vicious than his father, as he is visibly horrified when Morgan orders the execution of a young girl named Rika.

Helmeppo's motivations change drastically following Morgan's defeat by Monkey D. Luffy. Stripped of his father's protection and status, he is reduced to doing menial chores at the Marine base alongside another former captive, Koby. Initially, he resents this new life and rejects Koby's attempts at friendship. However, his perspective shifts when his own father takes him hostage during a prison transfer. Realizing that Morgan sees him only as a tool and is as brutal as any pirate, Helmeppo disowns his father and chooses to return to the Marines. From this point, his primary motivation becomes self-improvement and earning his place as a true Marine, leaving behind his spoiled past.

His role in the story evolves from a minor antagonist to a recurring ally within the Marine organization. After being taken under the wing of the legendary Vice Admiral Monkey D. Garp, he and Koby undergo intense training to become stronger and more capable officers. Helmeppo later reappears as a much-changed Marine lieutenant commander, having grown his hair into a ponytail, adopted a visor, and gained a more muscular physique. He participates in major events such as the Summit War of Marineford, and continues to serve alongside Koby, eventually becoming a member of the covert Marine unit known as SWORD.

Key relationships define much of his development. His relationship with his father, Morgan, is initially based on a delusion of mutual affection, which is shattered when Morgan physically strikes him and later uses him as a hostage. His bond with Koby is central to his redemption arc; they begin as reluctant coworkers but develop into close friends and trusted partners who push each other to grow. His relationship with Vice Admiral Garp is that of a strict but effective mentor who forces him to build real strength and discipline.

Helmeppo's development is one of the most significant transformations in the series. He goes from a weak, entitled bully to a dedicated and relatively humble Marine officer. He becomes self-aware enough to refer to his past self as "that bratty, selfish son who lived off his father's status." While he retains some comedic traits, such as being overly formal and easily flustered, he matures into a person capable of bravery and loyalty.

In terms of notable abilities, Helmeppo initially has no real combat skills, relying entirely on his father's name for protection. After his training under Garp, he becomes a skilled swordsman, specifically using a pair of kukri knives as his weapons of choice. He is also shown to have learned the six powers, a set of superhuman martial arts techniques, including the high-speed movement technique known as "Soru." Furthermore, he has demonstrated the ability to use both Busoshoku (Armament) Haki, which allows him to harden his body and weapons, and Kenbunshoku (Observation) Haki, which grants him enhanced spatial awareness.