The World Government in One Piece follows a highly structured power system with the mysterious Imu at the top. Although the higher figures are unarguably stronger than the lower-ranking officers, the hierarchy between the upper ranks is vague -- especially since the strength of the authorities above the fleet admiral remains a mystery. While it is clear that both the fleet admiral and the three admirals after him are forces to be reckoned with, vice admiral Monkey D. Garp can give them a run for their money.
Garp is one of the most beloved Marines in One Piece. Not just because of his association with Luffy, but also his overall carefree personality. Setting aside appearances, Garp's strength is nothing to scoff at. His feats as a young Marine earned him the epithet "Hero of the Marines," though he isn't fond of the incident accompanying the title. Garp also fought alongside and against big-name pirates during the legendary Gol D. Roger's era, earning more heroic tales under his name. With all his deeds and accolades, it seems only natural to wonder why Garp remains a vice admiral.
In One Piece Chapter 957, the former fleet admiral Sengoku detailed Garp's heroic past to the other Marine officers. He told them of the God Valley incident, where Garp teamed up with Roger to stop the infamous Rocks Pirates. Garp refuses to recall the incident not only because he fought alongside a pirate, but also because he fought for the Celestial Dragons' welfare. Sengoku further explained that Garp's moral compass won't allow him to serve the despicable World Nobles -- also the reason he turned down promotion on multiple occasions.
Even though the Marines are the supposed embodiment of goodness in One Piece, protecting the vile Celestial Dragons is its primary duty. Fans are well aware of the World Nobles' wickedness. After all, the Celestial Dragons have openly committed multiple crimes against humanity without batting an eyelid. Sengoku reiterated that the admiral's position is directly under the Celestial Dragons' command, and One Piece proved this by showing Fleet Admiral Akainu being at the Five Elders' beck and call. Garp is aware of this, and as a result he repeatedly rejected offers of promotion.
The Marine vice admiral has proved on various occasions that he doesn't share Akainu's version of justice, and vehemently believes that being a criminal does not necessarily equate to being a bad person. To prove this, Garp adopted the infamous Pirate King's son, Ace, as he agrees with Roger's view: a father's crimes are not inherited by the son. He also entrusted both Ace, his adopted grandson, and Luffy, his biological grandson, to the care of Curly Dadan, a mountain bandit. Similarly, Garp has no issue attacking a Marine officer for his personal interest, as shown when he was about to kill Akainu for murdering Ace.
Luffy may have inherited his grandfather's carefree attitude, but he also gained the vice admiral's firm morality. Garp despises turning a blind eye to the Celestial Dragons' despicable deeds, and more so serving under them. So even though he was repeatedly offered a promotion above vice admiral, he always turned it down -- and it is unlikely he will be changing his mind in future chapters of One Piece.
