SPOILERS!
Diamond is Unbreakable, or DiU is the 4th part of the Jojo's Bizarre Adventure saga. It follows Josuke Higashikata, a high school delinquent as he goes about his daily life in Morioh city. The story is split into several arcs, as per the norm for a manga, with the first arc being Josuke's experiences with other Stand users as he enters high school, meeting his closest friends Okuyasu Nijimura and Koichi Hirose along the way while hunting down the Bow and Arrow (mysterious artefacts that encourages the growth of a Stand in others) with the guidance of his older nephew, Jotaro Kujo, the protagonist of Part 3: Stardust Crusaders. The second arc chronicles his hunt for a killer in Morioh, Kira Yoshikage, who has a fixation with hands. It gets personal in an adventure and that leads to the final arc where they hunt down Kira in disguise while his dead dad, armed with a second Bow and Arrow seeks out several others to stop Josuke and his friends.
Josuke Higashikata is the main protagonist of this Part, and as per Jojo rules his name forms a JoJo (the suke in Josuke can be read as Jo). He's incredibly proud of his pompadour and has a hustler personality. His Stand, Crazy Diamond (a reference to the Pink Floyd song "Shine on you Crazy Diamond") has the power to fix anything, or return an object to its initial state. In the context of his team, he plays the part of Healer and DPS.
A contrast to Josuke's heroism, the main antagonist of DiU is Yoshikage Kira, a serial killer with a hand fetish. He has gone unnoticed for years, with his victims reported as missing. The team is alerted of him because of the ghost of one of his victims, and begin to infringe onto his "peaceful" life. His Stand, Killer Queen (a reference to the Queen song of the same name) allows him to turn anything it touches into a bomb.
DiU was the first JJBA part to be set exclusively in a single area, as all the events in this Part transpire in Morioh city, a departure from the previous stories which feature travelling, usually globe-spanning. The story also introduces a slice-of-life element in that Josuke goes to school, goes to cafes, makes friends all the while protecting his city from the Bizarre elements that goes on, which makes him the only Jojo protagonist to do so.
Araki (Jojo's Creator) has stated that DiU is his favorite part and Josuke his favorite protagonist, because in his opinion Josuke is the sort of dependable senior, a contrast to Jotaro who is more akin to a hero the world looks up to. This makes him relatable, a fact that I feel is crucial to his popularity.
In my opinion, DiU is not my favorite part, but Yoshikage Kira is a unique villain within the context of JJBA. Unlike previous villains such as DIO (remaking the world in his image) or Kars (become the greatest lifeform) or villains that come after him, Kira just wants a "quiet life". He wants to continue his lifestyle uninterrupted, just a shark in a lake. I think this is a great metaphor for monsters living among us, translated into this serial killer. No grandiose plans, no ridiculous motivations, just a desire to have things stay the way they are. This makes him relatable too, in the same vein as Josuke, making him a villain you can't help but root for.
Josuke however falls flat in my opinion precisely because of his down-to-earth-ness. He's a good protagonist, but not a great one in my book. No doubt about it, he's a tough cookie, but not a great guy to be around.
A little bit of trivia: The first fan translation of DiU is called "Duwang Scans", notable for their horrible quality and weird grammar. It serves as a hilarious companion to the series though, and I recommend it to anyone who's interested.
