One Piece: Marine Luffy, Garp regrets it Novel

Through isekai, a protagonist awakens as five-year-old Luffy and is inducted into the Marine under his grandfather Garp's command. Initially marked for military success, Luffy's upbringing becomes increasingly tumultuous, generating escalating regret in Garp as the years pass. His convictions prove fundamentally incompatible with Marine values: he champions the destruction of Celestial Dragons as justice incarnate, contests the laws preventing Marines from claiming kingship, and audaciously pursues multiple wives in blatant violation of Military tradition. Garp finally breaks, pleading desperately: "Grandson, cease this rebellion—or watch the Monkey family descend into the tyranny of Celestial Dragons!"