Diamond is Unbreakable

Diamond Is Unbreakable (ダイヤモンドは砕けない) is the fourth story arc of JoJo's Bizarre Adventure series.

The story focuses on Josuke Higashikata, the illegitimate son of Joseph Joestar, with Jotaro Kujo and Joseph Joestar along in a supporting role. Araki retroactively introduces the Bow and Arrow, an ancient artifact which causes latent Stand ability to develop in people.

Plot Summary
In Morioh Town, Josuke is one of the more popular guys in high school, and becomes quick friends with one of his classmates, Koichi Hirose. But when Josuke's "nephew" Jotaro (who happens to be quite a bit older than him) shows up, things get truly crazy. One of Dio's henchmen has brought the Bow and Arrow to Morioh, and someone has been using it on various people in the town--as discovered when one of them, the murderer "Angelo", uses the power of his water-Stand Aqua Necklace to get revenge on Josuke's grandfather for putting him in prison. When Josuke and Koichi go to investigate Koichi is struck by the arrow, unleashing his stand Echoes. They discover that the culprits are Okuyasu and Keicho Nijimura, whose father - formerly Dio's lackey, and injected with Dio's cells - has now turned into a monstrous creature with Dio's death. Keicho wants to create Stand users so that someone will be able to kill his monstrous father. Josuke manages to show the brothers that their father is still at least partially human inside, and dissuades them from their activities. However, they are attacked by the Stand Red Hot Chili Pepper, which kills Keicho and takes the Bow and Arrow. Okuyasu joins with Josuke, Jotaro and Koichi to fight the Stand that killed his brother, and eventually becomes close friends with Josuke as well. Only when Joseph Joestar, the father Josuke never knew, arrives in Morioh are they finally able to defeat Red Hot Chili Pepper. The Bow and Arrow are taken into Speedwagon's custody, and all seems to be over for the moment.

Koichi soon takes center stage when he is kidnapped by his psychotic classmate Yukako Yamagishi, who is in love with him and is determined that he will love her back. He escapes from her, and in the process his Stand evolves into a stronger form. Yukako is impressed, and decides to pursue Koichi in a more consensual but no less aggressive manner. Later Koichi meets manga author Rohan Kishibe (possibly a self-insert of Hirohiko Araki's), whose Heaven's Door ability allows him to open up a person's face like a book and both read and write their memories; Rohan finds himself drawn to the plucky Koichi, but does not get along with Josuke. Yukako and Rohan both become part of Josuke's band of Stand users.

Soon they meet the ghost of a young girl named Reimi Sugimoto, who was murdered by a serial killer over a decade before. She begs Koichi and Rohan to find her killer, who has stayed in Morioh and is still preying on innocent people. This killer turns out to be Yoshikage Kira, a serial killer who destroys women's bodies using his explosive Killer Queen stand and keeping the hands as ghoulish romantic objects. But when Josuke and Okuyasu's new friend, Shigechi discovers Kira's secret and is killed for it, they become resolved to put an end to Kira's depredations. Jotaro and Koichi eventually fight Kira, but before he can be fully defeated, he is able to force Aya Tsuji, the user of Cinderella, to alter his features, and kills her before she can tell Josuke what he now looks like. Shortly thereafter, the ghost of Kira's father deploys a second Stand-creating Bow and Arrow to create an army of Stand users to protect his hiding son, and the stage is set for a showdown of epic proportions between Kira and Josuke.

This series is most notable for taking place both in Japan and in an imaginary place, without any trips to real locations. (The plot is mystery-oriented, not travel-oriented; the characters try to locate Kira without being killed, but mostly they just wander around and encounter weird people.) Also, it really pumps up the rock references; there's even a villain jamming on a guitar. For its sheer inventiveness and its artwork, it occupies a high rank among many fans, even though it's probable that "JoJo's Bizarre Adventure" could have ended with Series 3, since killing Dio was the last thread that needed to be tied up from the preceding parts. Also sometimes when a character is defeated the character will "shrink" from being around Josuke's height as a villain to Koichi's height as an ally (i.e. Tamami Kobayashi, and Toshikazu Hazamada).

Trivia

 * According to Araki, the setting for Part 4 originated from the actual serial killings that had been happening around Japan at the time. Araki admits some of the enemy concepts in Part 4 were leftovers that he was not able to put into Part 3.
 * Part 4 is infamous for the Duwang translations; an English project by two Chinese students who were assigned to translate something into English as part of the assignment so they decided to translate Part 4, but failed the assignment due to how awful the translation is with its many translation errors such as new names for some characters (Shigekiyo Yangu is called Fatty), for changing the name of the main setting from Morioh to Duwang and for creating infamous translations such as 'What a beautiful Duwang' and 'Get a feeling so complicated'. Ironically these Duwang translations were the only English translation of Part 4 that English reading fans could read for a while.