One Piece: The Marine Ancestor

"I raised the Hero of the Marines, mentored the Fleet Admiral, and shaped the era itself. Now, at a hundred years old... my journey is finally beginning." Transmigrating into the world of One Piece with a personal System, Rowan expected to reach the pinnacle of power in record time. But fate had other plans—the System glitched, remaining stuck in a loading loop for decades. Rowan didn't wait around for a miracle. Using his knowledge of the future, he carved his own path. He adopted a rowdy brat named Monkey D. Garp, forged a life-and-death brotherhood with future Commander-in-Chief Sora, and disciplined a generation of legends including Sengoku, Zephyr, and Tsuru. A century has passed. While training his great-grandson, Luffy, Rowan finally feels the heavy toll of time on his bones. Just as he prepares to close his eyes and accept the end of his long watch, a familiar sound echoes in his mind: [System Loading: 100%... Activation Successful.] [First Mission: Become a Marine Admiral.] Looking at the prompt, the century-old legend can only scoff: "Are you kidding me? I’ve been the backbone of the Navy for a hundred years, and now you want me to start a career as an Admiral?!" The Old Guard has returned. The pirates think the era belongs to them—until they meet the man who raised their greatest enemies. — Description from novelcodex.org