Koichi Hirose

Koichi Hirose (広瀬 康一, Hirose Koichi) is a fictional character from Part IV: Diamond is Unbreakable. He is featured as one of the main characters and also makes an appearance at the beginning of Part V: Vento Aureo.

Diamond is Unbreakable (1999)
Koichi is a first-year high school student. At first, he seemed to fit the "best friend" spot, but he was quickly replaced by Okuyasu. Still, he remains well-involved with the overall plot, gaining a lot of credit in the chase against Kira, his stand even evolving multiple times as he fights a few battles on his own.

Throughout the series, he makes friends with Jotaro and Rohan.

Yukako, a fellow student in love with him, at first proved to be very difficult for him to deal with as she was very aggressive in her affections towards him - to the point of kidnapping him and threatening his life should he not appreciate her efforts to "change him into a better person". Later in the story, still lovestruck, she seemed to have learned from her mistakes in trying to force him and approached him more carefully, upon which Koichi warmed up to her and slowly, willingly, became her boyfriend.

Vento Aureo (2001)
Jotaro sends Koichi to Naples, Italy to locate Haruno Shiobana in order to get a skin sample for the Speedwagon Foundation to test. Shortly after arriving, Koichi notices Giorno performing in the street and then subtly bribing the policemen watching. Giorno offers to give him a ride, but Koichi refuses until noticing the long line of people waiting for a taxi. Unfortunately, the offer was a trick and Giorno starts to drive off with Koichi's luggage before he can get in the car. Using Echoes ACT 3, Koichi stops the car but Giorno simply smiles and flees the scene after transforming the luggage it into a frog. Koichi overhears the nearby policemen talking about Giorno's failure and realizes that "Giorno Giovanna" is actually the "Haruno Shiobana" he'd been sent to find. He doesn't, however, figure out that the frog he found in the back of the car is actually his luggage.

Some time later, when Giorno is undergoing the test to join Passione, Koichi locates his apartment and searches for his missing luggage. He nearly extinguishes the lighter flame which Giorno had stood in a loaf of bread, but Giorno whisks it away with Gold Experience just in time. Having successfully found his passport, Koichi leaves the apartment building only to stumble upon Giorno and a dead janitor. At first he believes Giorno is responsible, but soon learns that it was Black Sabbath's doing. He ends up helping Giorno to defeat the stand, noticing in the process that it possesses part of the Bow and Arrow. Koichi then accesses a payphone to inform Jotaro, but Giorno stops him and Koichi decides to remain quiet in deference to Giorno's virtuousness and behavioral similarity to those of the Joestar bloodline.

Personality
Koichi is a friendly, rather naive boy. He is perfectly willing to help strangers but doesn't seem to suspect they could have bad intentions, which tends to land him in trouble. Despite his naivety, he has a sense of determination when things which are important to him are on the line. He has also shown himself capable of fighting against his own fears for the greater good, like when he went to retrieve the Bow and Arrow despite being terrified and when he hunted down Giorno to retrieve his stolen passport and luggage.

Appearance
Despite his age, Koichi is very short and is often drawn dwarfed by taller characters like Josuke and Jotaro. The height difference doesn't seem consistent, however, as seen when he first encounters Jotaro: when they bump into each other, Koichi is about as tall as Jotaro's waist, but when they pass some delinquents, he is almost as tall as Jotaro's shoulder. Prior to developing a stand, Koichi's hair is brushed back and rather tame. Afterward - particularly once his stand evolves - his hair takes on a more spiked appearance, similar to that of a Super Saiyan.

Trivia

 * When Koichi visits Italy in Part V: Golden Wind, he indicates that he asked Rohan to write 'Italian language skills' into him with Heaven's Door.
 * He owns a dog named Police, a reference to the band of the same name.
 * He is the only non-JoJo character to meet 4 JoJos throughout the adventure ( Josuke, Jotaro,  Joseph and  Giorno).
 * He is also one of the few non-JoJo characters to take part in more than one adventure. Other examples are Jean Pierre Polnareff, Robert Edward O Speedwagon, Straights, Erina Joestar, and Suzie Q
 * His JoJoLion counterpart is a girl called Yasuho Hirose.