Hows this?
He may be serious, but Jimbei has been shown to have fun, and has some snappy comebacks. But more importantly, he gave up everything for his home and friends ten times over. He spent time with the Sun Pirates to protect his foster brother Fisher Tiger, took the title of Warlord to pardon the former slaves who never wanted to be pirates and to strengthen the bonds between humans and Fishmen, gave up that title to try and save Ace and Whitebeard, stared down the Red Dog in order to protect Luffy, helped Luffy through the loss of his brother, forced to ally himself with Big Mom to help his crew protect Fishman Island, was willing to accept any punishment for the pain he had unintentionally bestowed upon Nami (which she forgave Jimbei for), gave blood to save Luffy's life, and who knows what else he'll sacrifice. I just hope it's not his life. He needs to honor the promise he made Ace and protect his little brother. Best way for Jimbei to do that is to join the Straw Hat crew.
Trafalgar Law started out as just another one of the Eleven Supernovas, albeit I thought he had the best design and DF, but I thought he would only be one of the many one off enemies of Luffy's. Little did I know that for reasons known only to himself (may be related to this Cora of whom he spoke), Law chose to go out of his way to enter the biggest war in the world to save Luffy from the Admirals, in his own personal sub. He treated Luffy and Jimbei's injuries, then left without even saying goodbye, noting that the D would whip up another storm. In the timeskip, he unfortunately got rid of his Pre TS hat. But the plot twists that followed more than made up for it. Law became a Warlord to try and bring down Doflamingo, had one of the best fights in the series with Smoker, oneshotted Doflamingo's right hand man (with an epic "in your face" speech), and kicked off the events leading up to Dressrosa. Instead of taking out Kaido, turns out he was aiming for Doflamingo all along, and was willing to die to make it happen. I think Law will face consequences for his actions, but there's no doubt in my mind he'll continue to work alongside Luffy, as I believe he's Luffy's own Mihawk (just compare Luffy and Law to Shanks and Mihawk, you'll see what I'm talking about).