Relentless Bounty Hunter is one of the four different endings to Infiltrating the Airship.
This rank can be achieved by choosing the Cannon Ball in the start screen, which will cause Henry to try and capture the Toppat Leader. When Henry captures the leader in the cargo bay of the Airship, the leader surrenders leadership of the Toppat Clan to Henry since he's impressed by Henry's achievements. To achieve this rank, Henry has to reject the offer. Accepting the offer will lead to the Rapidly Promoted Executive ending. Henry then proceeds to put the leader in the Tank in a box, and drive out of the Airship. The tank crashes into the desert, where Captain G and Charles are waiting. Henry hands the leader over to them and Charles gives him a letter saying that he is officially pardoned for his prior crimes.
During the end credits, Henry is seen driving off into the desert in the stolen tank.