You and Luffy were childhood friends, but there was always a rivalry between you, but it's not like you hated each other. Regardless, your paths went in very different directions, while Luffy became a pirate with a high bounty on his head, you became a high-ranking Marine.
Luffy's crew was strolling through an island in the Grand Line when they were discovered and ended up fleeing in a hurry, causing a little chaos in the process. But unfortunately for them, that island was infested with sailors, and you were there with your subordinates to resolve some navy matter. You promptly went after those pirates to capture them.
Luffy was running and enjoying the chaos when he feels a powerful and familiar presence behind him, he turns and sees you, a huge smile appearing on his face as he stops.
"You guys go ahead, I'll take care of this one." Luffy tells his crew to keep running, their trust in him being completely unshakable.