JoJo's Bizarre Adventure keeps itself fresh by regularly rotating through its protagonists and settings. In Part 4, Diamond is Unbreakable, the hero was Josuke Higashikata, a small-town punk with a good heart. Readers may be reminded by another punk with a good heart: Yusuke Urameshi of Yu Yu Hakusho fame. The Yu Yu Hakusho manga ran from 1990 to 1994, roughly parallel to Diamond is Unbreakable's 1992-1995 run, and the similarities between the two series are remarkable.
Josuke & Yusuke: Separated At Birth?
The main characters of these two series have a startling amount of overlap, in terms of their fighting style, personality, life situation and more. In Yu Yu Hakusho, Yusuke is a delinquent with a heart of gold who gave his life to save a child from a speeding car, thus ending up in the afterlife. He was given another chance at life by Jr./Koenma, who granted him the rank of spirit detective.
Of course, Yusuke was ultimately more of a fighter than a detective, battling his way through such adventures as the Dark Tournament and going to the Demon Plane to face his destiny at last. Yusuke had street smarts and leadership qualities, attracting a small gang of friends including enemy-turned-ally Kazuma Kuwabara, scheming demon Hiei and the plant-controlling demon fox Kurama. Yusuke usually let his fists do the talking, and he wasn't a fan of fancy rhetoric or mincing words.
Josuke Higashikata has a lot in common with Yusuke, beyond their shared "-suke" male name suffix. Just like Yusuke, Josuke is a tough kid who has natural charisma as a leader. His own squad is made up of Okuyasu Nijimura, Koichi Hirose, Shigechi (for a time) and, in a distant sense, Rohan Kishibe. Josuke doesn't put up with anyone's nonsense, and he'll get riled up if anyone makes fun of his carefully-styled hair. Like Yusuke, Josuke is a hero at heart, and it's likely that he once went back in time to save his mother and younger self from a snowstorm while they were driving to the hospital (a car-related parallel to Yusuke). Josuke also lets his fists do the talking since he doesn't really have a way with words. His charisma comes from his actions and his spirits, not flowery speeches.
Josuke & Yusuke's Bizarre Journeys
The parallels between Josuke and Yusuke's personalities don't necessarily mean much, but the story similarities between JoJo and Yu Yu Hakusho extend beyond that. Both Yusuke and Josuke are living with their mothers, having a respectful but otherwise unremarkable relationship with them. Both mothers are tough and capable women who can keep up with their son's antics. Yusuke has no father figure in his life, and Josuke has never met his own father, the aging Joseph Joestar, but both boys will come face to face with a father figure before long.
In the final arc of Yu Yu Hakusho, Yusuke visits the Demon Plane and meets his forefather Raizen, having a rough start to their relationship before working things out fist-to-fist. Josuke, meanwhile, finally meets Joseph halfway through his story arc, and at first, Josuke is wary of him. After they go through a pretty bizarre adventure of their own, they end up on much better terms.
Both heroes also fight a thuggish enemy, then makeup with said opponent and befriend him (and they might even develop a slight Vegeta complex). Yusuke clashed with Kazuma Kuwabara at first, but the boys shook hands and made up, and Kazuma became one of Yusuke's best and most reliable allies. Josuke had a similar experience with Okuyasu Nijimura, who wields the Stand called The Hand. He was hostile at first, but he and Josuke set aside their harsh feelings before too long, and Okuyasu actually approached Josuke later with overtures of friendship (which Josuke accepted). Like his counterpart Kazuma, Okuyasu became a dim-witted but loyal and reliable ally.
Though the similarities are obvious, Diamond is Unbreakable is not a rip-off of Yu Yu Hakusho. Both JoJo's Hirohiko Araki and YYH's Yoshihiro Togashi are colleagues who both draw inspiration from one another. Just as JoJo borrowed character types from YYH, Togashi would, in turn, be influenced by JoJo's Stand abilities in developing YYH's Chapter Black Saga and the Nen abilities in Hunter X Hunter. Pretty much if you love one author's work, you should definitely check out the other's.