Koby first appears as a timid, bespectacled cabin boy forced to work on Alvida’s pirate ship. His early life is defined by fear and servitude, having been conscripted into piracy against his will. He dreams of joining the Marines, but his lack of confidence and physical weakness make that goal seem impossible.

Personality-wise, Koby is initially anxious, tearful, and prone to panicking. He is deeply empathetic and cannot tolerate cruelty, even when he feels powerless to stop it. Despite his fear, he possesses a strong moral compass and a growing sense of justice. Over time, he becomes more disciplined, courageous, and outspoken. He retains his politeness and idealism but sheds his cowardice, transforming into a resolute young man who upholds Marine integrity.

Koby’s primary motivation is to become a Marine admiral. This dream is not born from a desire for power but from a wish to stop needless suffering and piracy at sea. He wants to protect the innocent and enforce a fair kind of justice, contrasting with more absolute or corrupt Marine ideologies.

In the story’s narrative, Koby serves as a parallel and a foil to Monkey D. Luffy. Their first encounter at sea leads Luffy to accidentally help Koby escape Alvida, and Koby joins the Marines shortly after. He reappears at key points—such as the Post-Enies Lobby arc and the Marineford War—to demonstrate how regular people can grow through relentless effort. His progress also offers a ground-level view of the Marines, showing both their noble ideals and their institutional flaws.

Key relationships define much of his development. His bond with Luffy is one of mutual respect despite their opposing affiliations; Luffy calls him “Coby” (intentionally misspelling his name as a nickname) and treats him as a friend, not an enemy. Koby considers Luffy a rival and inspiration. Another significant relationship is with Vice Admiral Garp, who takes Koby under his wing alongside Helmeppo. Garp’s tough training and belief in Koby’s potential are catalysts for his physical and mental growth. Koby and Helmeppo start as unlikely peers—Helmeppo originally being a spoiled bully—but develop a close, brotherly camaraderie as they endure Garp’s training together.

Koby’s development is one of the most pronounced in the series. He begins as a helpless captive, transforms into a Marine recruit with shaven head and leaner build, and later becomes a confident officer. By the time of the Summit War of Marineford, he momentarily stops the fighting with a desperate, empathetic cry, displaying newfound authority and moral clarity. In post-timeskip storylines, he has risen to the rank of Marine Captain and later Rear Admiral, demonstrating leadership and tactical thinking.

Notable abilities include Koby’s latent talent for Rokushiki, a superhuman martial art used by elite Marines. He masters techniques like Soru (movement) and Geppo (air jumping) through intense training with Garp. He also develops Kenbunshoku Haki (Observation Haki) to a highly refined degree, allowing him to sense the presence, emotions, and intent of others over a wide area. This Haki is powerful enough to perceive threats at a distance and contributed to his instinctive intervention during Marineford. As of later story events, he has also shown signs of developing Busoshoku Haki (Armament Haki). While not a Devil Fruit user, his growth in physical combat, Haki, and Marine tactics makes him a formidable officer who embodies the potential for genuine change within the World Government’s military structure.