Marcoh is no ordinary guy. Why do you think he always keeps his back covered by his combat jacket? Why do you think he is always so reserved around most people? Because he is secretly a Joestar. Not by direct lineage but he has some blood running through his veins. Think about George Joestar the First, Jonathan's father, he had only one child, but that perhaps because his wife passed away early on and he didn't wanna remarry or couldn't get past her death. So, Jonathan is an only child, yes, he is. But what about his dad? Considering that he grew up in the early Victorian era, customs at the time were to have more than one child hence a lot of them didn't make it through childhood due to illnesses and the lack of a structured medical system at the time. Look at Great Expectations, in the first scene, Pip is at his parents' grave and reflects on their passing, while acknowledging he also had other siblings who bit the dust, only he and his older sister remaining alive. So. its a very high possibility George Joestar had more siblings, some may have passed, some may have lived on and one of them could have moved with the trading business or got married in Italy. The family could have adapted to the Italian language or maybe after the first generation, only girls were born so they got married and changed their name entirely. And keep in mind that its not compulsory that George Joestar is the oldest sibling, he could very well be the youngest of them and have much older siblings, some who might have even passed away during the first act of Part 1. Marcoh was born at the beginning of the 1910s so there was plenty of time for the other Joestars to settle and blend in Italy considering we are talking about Victorian or Pre-Victorian(possibly) era people. Besides his physique and height which run in the men of the Joestar clan, we can also notice that Marcoh has an innate appeal to people who want to be stronger. Basically, its not a stretch to call Tanaka his Jobro. Also, why is Marcoh so physically strong? I mean, if you take it literally, he can punch divine-like beings and do damage to them. I am inclined to believe Marcoh has secretly mastered the art of Hamon or maybe he even posses a Stand that nobody else can see, not even us, because we are not Stand Users, nor are any of the other characters. Think about Marcoh's Counter skill, is it simply that he can counterattack that fast? or is that his stand reacting? What about the red aura that engulfs him the more a fight goes on, could that simply be a cool visual effect, or his Hamon energy experiencing an overdrive like Jonathan did? But hey, thats just a theory! A Funger Theory!