Law is the only supernova next to luffy that is a D. He was luffy equal partner in the alliance to take down yonkous next to luffy. Both got equal credit for defeating doflamingo and same bounty, both tanking down yonkous now law vs Linlin, luffy vs kaidou.
Law wants to find one piece so he can find the full history of D. , Luffy wants to find op for the treasure, so they will fie IT together and law will be vice pirate king