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 he's depicted as shorter than that. He wears a notably unmodified student uniform, otherwise preferring leisure wear.

Koichi first keeps his light hair neatly combed back. During his confrontation with Tamami, 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 his confrontation with Yukako 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
While typically humble and soft-spoken, Koichi undergoes an exceptional degree of character development in Part IV, growing consistently into greater self-confidence as he is challenged by threats and anxieties 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. By the end of Part IV, he displays immense bravery and determination after his encounters.

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 virtuousness and state their pride at his friendship, including Josuke Higashikata, Rohan Kishibe and Jotaro Kujo. Others tentatively brought from deficit into moral equilibrium by his influence include Toshikazu Hazamada, Tamami Kobayashi, and Yukako Yamagishi; his eventual girlfriend.

Abilities
Koichi's Stand, Echoes, takes three forms: ACT 1, ACT 2 and ACT 3.

ACT 1 and ACT 2 enable him to give offensive substance to sounds of his creation.

ACT 3, a humanoid Stand, facilitates conventional melee attack and the ability to "freeze" targets by ascribing them unbearable weight.

Jotaro Kujo! Meets Josuke Higashikata
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.

The Nijimura Brothers
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 Hirose (Echoes)
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.

Toshikazu Hazamada (Surface)
With the help of Tamami, Koichi finds the stand user Toshikazu Hazamada and is quickly defeated, but still befriends him too.

Yukako Yamagishi Is in Love
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.

Red Hot Chili Pepper
Akira Otoishi reveals later that Koichi was one of the stand users he watched with his Red Hot Chili Pepper.

Let's Go Play at the Mangaka's House
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 Kishibe's Adventure
Rohan and Koichi accidentally find Reimi Sugimoto's alley and learn about the killer Yoshikage Kira.

Yukako Yamagishi's Vision of Cinderella
When Yukako searches for Aya Tsuji's help, Koichi is beginning to be attracted to 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.

Sheer Heart Attack
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 them alive from Killer Queen's Sheer Heart Attack ability. He was nearly killed before Jotaro was able to immobilize Kira.

Atom Heart Father
Later, Koichi, along with Josuke, Okuyasu and Jotaro search Kira's house, but the group is attacked by Kira's father, Yoshihiro.

Highway Star
During Josuke's encounter with Highway Star, Koichi helps locate Yuuya Fungami by tracking his stand.

Enigma Boy
Koichi is kidnapped by Terunosuke Miyamoto, whom traps him in a piece a paper with his stand, Enigma. His life is put into danger when Terunosuke throws Koichi in front of a car, but he ends up being saved by Yuuya.

Cheap Trick
Koichi also tries to save Rohan from Cheap Trick, but almost rips out Rohan's back in the process.

The Invincible Crazy Diamond
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 considered canon.

After the events of Part IV, Koichi and Rohan find a cat (named Trinita) covered in blood and the two try to find his master. Koichi soon discovers that Hanae Orikasa, the cat's master, was killed under mysterious circumstances and tags along with Rohan to find the killer. After taking Trinita from the police station via Echoes, they discover, with the help of Heaven's Door, that the killer had a very distinct characteristic: red marks on his arms. Koichi 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.

Gold Experience
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.

Gang Enrollment
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.

Golden Whirlwind (PS2)
Koichi was planned to be on the GioGio PS2 game, most probably as playable character, since there are beta pictures of him (such as Koichi appareaing together with  PART V' s main characters and another one 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, such as Rissoto Nero or Leaky-Eye Luca, who does not appear on story mode but their characters model remains viewable).

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

All-Star Battle (PS3)
Koichi makes his debut in All-Star Battle as part of the PART IV  cast of playable characters. He was confirmed along with Akira Otoshi. Koichi is voiced by Romi Park, who previously voiced both Giorno Giovanna and Gold Experience Requiem in the aforementioned PS2 GioGio game, coincidentally.

In gameplay, Koichi uses all three forms of Echoes as part of his moveset, unlike Johnny or Pucci, who need to evolve their stands during combat. ACT 1 and ACT 2's powers serve as projectile-attacks like launching kanjis to hit the enemy or making an special effect such as burning, as well as throws and physical attacks. ACT 1 also appears as Koichi's HHA, assaulting his opponent with a loud barrage of sound. Echoes ACT 3 is used as Koichi's GHA as he knocks the enemy into the air, beating them up, and finally slamming them to the ground with Three Freeze, which also creates a slow effect on the enemy. ACT 3 can also use a Barrage Fist similar to Jotaro's oraora, but it does not work in Rush Mode. Koichi is also one of the smallest characters on the game, along with Iggy and Dismounted Johnny, making him hard to hit.

Aside from being a playable character, Koichi also appears on the stage "Kira Estate" as a background character along with Okuyasu. Obviously if Koichi is one of the two fighting characters, he will not appear on the stage's background. Koichi also appears as the auto-save icon, being affected by Heaven's Door.

Aside from the entire PART IV cast, Koichi possess a special dialogue before the fight with Jotaro and Giorno, as in the last one he says "I want my suitcase and passport back!" and Giorno replies "Could that be...?", recreating their first meeting on the first chapters of PART V.

Koichi also won the first place on his League Group on All-Star Battle League, beating fan favorites such as Giorno, Jolyne and Gyro. And even managed to reach to the semi-finals matches, but ultimately lost to Jotaro.

Koichi also posses two alternate costume, the first being the pajamas he wore during the LOVE DELUXE events and his second one being his T-Shirt with the LUCKY.CO logo on it that he wore during the fight against Highway Star.

Eye's of Heaven (PS3/PS4)
Koichi was confirmed for the game alongside Yukako.

Once again Koichi is voiced by Romi Park and his main color scheme it's the same from the previous 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.
 * Koichi's surname, "Hirose," is derived from the Hirose River (広瀬川), located within the city of Sendai, Araki's birthplace.
 * 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 Suzi Q.
 * His most probable JoJolion counterpart is a girl called Yasuho Hirose, due to her last name and the fact she's an ally.
 * Although it's never been explained why exactly, usually Koichi's enemies tend to shrink in size after being beat by him.