Koichi Hirose

Koichi Hirose (広瀬 康一) is a central character and ally featured primarily in Part IV: Diamond is Unbreakable.

Appearance
Koichi is a teenage boy of average build, 157 cm (5'2") tall, though depicted as shorter. He possesses a general facial neoteny and frequently raised eyebrows, effecting an innocent expression. He usually wears a notably unmodified student uniform; otherwise preferring leisure wear.

Koichi first keeps his light hair neatly combed back. It gradually ruffles, and during his confrontation with Yukako Yamagishi it gains the tendency to rise into spikes when he is excited (producing a look similar to that of Dragon Ball Z character Gohan at Super Saiyan 2during the Cell Games). Towards the end of this episode he suddenly cuts his hair, producing a flat-top (which spontaneously grows out again into spikes during his battle with Yoshikage Kira ).

Heaven's Door reveals that Koichi was born prematurely.

Personality
Koichi undergoes an exceptional degree of character development in Part IV, growing consistently into greater self-confidence as he is challenged by anxieties, fears and threats of widely varying degrees through numerous confrontations, including one critical battle with primary antagonist Yoshikage Kira. His greatest leaps are illustrated by the metamorphosis of his Stand, Echoes.

Koichi is intelligent, diligent and amicable. He expresses love for his family, holds his friends in esteem, and tends to presume innocence on the part of strangers. Heaven's Door exposes Koichi as honest and courageous.

Several characters praise Koichi's particular virtuousness and state their pride at his friendship, including Josuke Higashikata, Rohan Kishibe, Yukako Yamagishi and Jotaro Kujo. Others tentatively brought from deficit into moral equilibrium by his influence include Toshikazu Hazamada and Tamami Kobayashi.

Diamond is Unbreakable (1999)
Koichi is a first-year high school student. At first, he was saved by Josuke Higashikata from delinquents and seemed to fit the "best friend" spot. Koichi is shot with the Bow and Arrow by Keicho Nijimura, who was searching for a stand capable of killing his father. Koichi then developed the egg-shaped stand Echoes, whose ability was still unknown. Koichi tried to help defeat Keicho, but couldn't control his stand.

Koichi remains well-involved with the overall plot, his stand even evolving multiple times as he fights a few battles on his own. He is victim of the con artist Tamami Kobayashi, who threatens his family in exchange of money. In this battle Koichi develops Echoes ACT1 and defeats Tamami, who later befriends him. With the help of Tamami, Koichi finds the stand user Toshikazu Hazamada and is quickly defeated, but still befriends him too.

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. Koichi tried to look like a "bad guy" to keep her away but this plan backfires as she goes to the point of kidnapping him and threatening his life should he not appreciate her efforts to "change him into a better person". Koichi defeats her in battle after developing Echoes ACT2, but saves her life in the end. Akira Otoishi reveals later that Koichi was one of the stand users he watched with his Red Hot Chili Pepper.

Alongside Toshikazu, Koichi goes to the house of famous mangaka Rohan Kishibe to ask for an autograph. Even though Koichi gets scared by his actions to make his manga look realistic (like cutting open a living spider to see its insides), Koichi and Toshikazu sneak into his office to see his original drawings and are affected by Heaven's Door as a result. Rohan plans to use Koichi as a character for his manga, ripping pages of him every day. He is saved by Okuyasu and Josuke, who defeats Rohan after having his hair insulted. Rohan also ends up befriending Koichi.

Rohan and Koichi accidentally find Reimi Sugimoto's alley and learn about the killer Yoshikage Kira. When Yukako searches for Aya Tsuji's help, Koichi is initially attracted by Yukako. He even offers to become blind in case he fails to help Yukako return to her old appearance (changed by Cinderella). 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.

With Jotaro Kujo, Koichi finally encounters the killer Yoshikage Kira and develops Echoes ACT3. He gains a lot of credit in the battle, as his courage is fundamental to keep then alive from Killer Queen's Sheer Heart Attack ability, However, he was nearly killed by Kira before Jotaro immobilized Kira. Josuke and Okuyasu arrives at the scene & heals Jotaro & Koichi. Koichi, along with Josuke, Okuyasu and Jotaro ended up meeting Kira's father, Yoshihiro Kira, but Koichi couldn't help them because he was outside of Atom Heart Father's depicted area with Okuyasu. He also helps Josuke locate Yuuya Fungami by telling him recent news from where Highway Star was found.

Koichi is kidnaped by Terunosuke Miyamoto, who scared him with a scream so his stand Enigma could trap him in a paper. His life is put into danger when Terunosuke throws Koichi in front of a car, but he ends up being saved by Yuuya. Koichi also tries to save Rohan from Cheap Trick, but almost rips out Rohan's back in the process. Koichi finally finds Yoshikage Kira again and is fundamental to his defeat, as he freezes Kira's finger before he could activate his Bite the Dust ability. After Kira's death, he then sees off Reimi as she, along with her dog goes to heaven.

The Book: 4th Another Day
The information below derives from a Light Novel not written by Araki. As such, it may or may not be consideredcanon.

Koichi finds the cat Trinita covered in blood with Rohan and tries to find his master. Koichi discovers that Hanae Orikasa, the cat's master, was killed and tags along with Rohan to find the killer: a student with red marks on his arms. Koichi uses his stand to take Trinita from the police station so Rohan could use Heaven's Door on him and finds out that the killer is a student with a dinstinct characteristic: red marks on his arms. He later works with Yukako to discover if the killer is from the same class as them.

Rohan at the Louvre
Koichi makes a brief appearance, hearing Rohan's story about Nanase Kishibe along with Okuyasu and Josuke. Koichi questions Okuyasu about his opinion on Rohan's likeness with Mona Lisa, as they have different genders.

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. Afterwards, Koichi heads on home since Jotaro calls off the search as his prediction was correct.

Giogio's Bizarre Adventure: Golden Whrilwind (PS2 Game)
Koichi was planned to be on the Giogio PS2 game, most probably as playable character (there are pictures of him along with PART V's main characters and looking for Giorno much like he did on the first chapters of the manga). However, for unknown reasons his appearance on the game was dropped out (not even as character model on GALLERY mode unlike other characters who does not appear on story mode but has their character model like Risotto Nero and Leaky-Eye Luca).

During PART V manga, Koichi does have a fight against Giorno and latter he helps Giorno against Black Sabbath. Since he was absent from the game, during chapter 3 Giorno faces and defeats Black Sabbath by himself

JoJo's Bizarre Adventure: All-Star Battle (PS3 Game)
Koichi makes his debut in All-Star Battle as part of the PART IV  cast of playable characters. Gameplay-wise, Koichi uses all three forms of his stand, Echoes, as part of his moveset (unlike Johnny or Pucci who need to evolve their stands during combat). Echoes ACT 1 and ACT 2's powers serves as projectile-attacks (launching kanjis to hit the enemy or making an special effect such as burn effect), throws and "stands" physical attacks, ACT 1 also appears as Koichi's HHA ( assaulting his opponent with a loud barrage of sound). Echoes ACT 3 is more used as Koichi's GHA ( knocking the enemy into the air, beating them up, and finally slamming them to the ground with his increased gravity, also creating a slow effect on the enemy)  and Barrage Fist (Similar to Jotaro's oraora, but it does not work as Rush Mode).

Koichi is also one of the smallest characters on the game, along with Iggy and Johnny (Dismounted), making him hard to hit. Also, in ALL-STAR BATTLE Koichi is voiced by Romi Park, who previously voiced Giorno Giovanna in the Giogio PS2 game.

Trivia

 * When Koichi visits Italy in Part V: Vento Aureo, he indicates that he asked Rohan to write 'Italian language skills' into him with Heaven's Door.
 * Koichi's name and basic appearance appear to have been taken from the narrator of Chapter 1 of one of Araki's earlier works, Gorgeous Irene.
 * 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 series (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 include Jean Pierre Polnareff, Robert Edward O Speedwagon, Straizo, Erina Joestar, and Suzie Q.
 * His JoJolion counterpart is a girl called Yasuho Hirose.
 * Interestingly, Koichi and Giorno Giovanna share the same seiyuu, Romi Park, in different games. Romi Park dubbed Giorno in the Part V PS2 game, and Koichi in All Star Battle.
 * If Koichi fights Giorno in JoJo's Bizarre Adventure: All-Star Battle, they will have a special dialogue before the fight, where Koichi says "I want my suitcase and passport back!" and Giorno replies "Could that be...?", recreating their dialogue in the first chapters of Part V: Vento Aureo.