If you remember my pitifully short chapter on Hamon in a Vampire World, I basically just suggested that somebody should be reincarnated as an Ultimate Life-form with all knowledge of the Ripple in any setting which involves Vampires. At the time I couldn't think of where to throw a Hamon user or who to use but now I know what character and what setting. So, here we go with a proper idea for a fan-fiction. (I'm going to use Ripple and Hamon and Jonathan and Jojo interchangeably)
To start off, I'll give a reminder of the events of Phantom Blood: Jonathan Joestar is born as part of the wealthy Joestar Family in Britain during the late 1800's. Later, his father, George Joestar, adopts Dio Brando to repay his savior, Dario Brando, but he misunderstood the reason why he was saved. Dio torments Jonathan during their youth, culminating in Dio's attempts to steal Erina, Jojo's childhood love interest, which ends with Dio getting beaten down. Fast-forward to their college years, Jojo and Dio are seemingly on better terms, but that doesn't last as George's health has been declining. Lo and behold, Jonathan discovers that Dio has been poisoning George, as he had done to Dario before. Meanwhile Dio figures out that the stone mask that had been part of the decorations in Joestar Manor could turn people into vampires, and he formulates a plan. Upon arriving back home, Dio is surrounded by police, and while being handcuffed attacks Jojo, but George protects him, dying as a result. Dio becomes a vampire, and he and Jonathan fight, with Jojo ending up the victor and killing Dio, or so he thought. Recovering from his injuries with help from Erina, Jon meets with Will Anthonio Zeppeli, who tells him that Dio is still alive and that he wants to teach him Hamon. Jonathan turns out to be a genius in the Ripple, picking up what Zeppeli learned in decades in mere days. During the fight with an incredibly strong zombie, Will gifts Jojo with all of his Hamon, causing his strength to skyrocket. Eventually, Jonathan gets his rematch with Dio, which he wins, but once again he fails to finish off the immortal being. Jonathan is finally killed on a ship, and Dio takes his body, to sleep for a century when the events of Stardust Crusader begin.
Okay, long spiel out of the way, let's talk about why I chose Jonathan instead of Joseph. And it mainly comes down to, Joseph had a much longer life than Jonathan, he had opportunities to see his grandchildren, he had a daughter and later a son. Jonathan probably didn't even know Erina was pregnant, and I think he deserves a second chance. I mean, look at it like this: Joseph at his oldest was 79, whereas Jonathan at his oldest was 20. See my point? Oh, and before you mention it, I am aware of others introducing Hamon to this world, but I'm more interested in the characters than the techniques.
So, now we move on to how Jonathan makes his way to Demon Slayer. After his death on the ship, Jojo finds himself before God, who pities his short existence and decides to give him a second chance, letting him keep his memories and sends him to a new world but not before warning him of a threat similar to Vampires. Unbeknownst to them, part of Jonathan's soul is still trapped within his, no, DIO's body. (It's there as a way to give him memories of Stardust Crusader's events when DIO dies) He should regain consciousness either in the womb or once born and immediately begin training his Hamon. I suggest that he be born to a duplicate family, who will instill the same morals in him as the original. He should be reborn maybe 15 or 20 years before the plot begins. Now, what reason could there be to send him to Japan? Perhaps a family trip, or maybe a business venture? No, his memories from the other part of his soul trapped in DIO makes him desire to see the land that birthed his great-great grandson.
When he arrives, he hears tales of an organization that hunts down beings known as Demons. Understanding what kind of danger could befall the innocent he decides that it would be best if he offers help. Later on in the story he could meet with Tsugikuni Yoriichi and learn the Breath of the Sun. I wouldn't accept any of the other Breath Styles, as they don't hold the same relation to Hamon that the original Breath would. As for how a Nichirin Blade would react to the Ripple, I think it would be a similar function as the Red Stone of Aja, where it would conduct and amplify the Hamon. Now, what color would the blade turn is a question for the Author who picks this up, but my thoughts are that it would turn black, but I'd think it would be cooler if it turned golden or some other unique color.