Rogge did not know much about what Dragon had done back then, but he had managed to uncover a few details. It had happened in Mary Geoise.
Since the incident involved the Celestial Dragons, it was perfectly normal that Rogge knew nothing about it. Even many veteran Marines were unaware. Only by bribing World Government officials had Rogge learned about the past from soldiers who had served in Mary Geoise.
But they were only minor figures, after all, and had access to very little. What Dragon had done back then had indeed enraged the Celestial Dragons. If Garp had not been a Marine hero, he probably would have suffered when they failed to find Dragon.
Dragon's strength spoke for itself. Rogge's fists and his Dragon Claw Fist were almost evenly matched.
After all, he had trained under Garp for more than twenty years. The training Dragon had endured was even more terrifying than Luffy's. Grandparents doted on their grandchildren, and however Garp treated Luffy, love lay behind most of it. With Dragon, he had wanted his son to amount to something. So that shoehorn-shaped tattoo on Dragon's face...
Just kidding!
It was precisely because of Garp's strict demands that Dragon's Devil Fruit abilities and physical combat skills ranked at the very top among the younger Marines of roughly his age. Even Sakazuki and Borsalino, who was already in his twenties, could only eat his dust.
After Roger's death, the Marines had even planned to fill the third Admiral position. Since Garp did not want it, Dragon would become the new Admiral. Once Sengoku became Fleet Admiral, Dragon would continue serving as an Admiral, then succeed Sengoku as Fleet Admiral.
The Marines should originally have had four Admirals, but this dragon had flown away. They had agonized over the matter for