Koya is a Pokémon Trainer and a member of the International Police, a law-enforcement organization that tracks down the various evil organizations in the Pokémon world. In terms of personality, Koya is cold and does not hesitate to show his opinion of someone, be it negative or positive. Despite his cold nature, he truly loves Pokémon and joined the International Police to apprehend those that would harm Pokémon.
It is revealed that in the past, he was once like any regular Pokémon Trainer, but a crushing loss traumatized his beloved Growlithe. Koya felt that this was his fault and resolved to prevent other Pokémon from suffering the same fate, which was his motivation for joining the International Police.
|Spoiler warning: this article may contain major plot or ending details.|
Koya first appears as a competitor of the Sinnoh League where he is seen witnessing the first round of the tournament. When Hareta notices his Absol and tries to touch it, Koya coldly punches him for trying to touch his Pokémon without permission. Sensing that he is strong, Hareta asks Koya for a Pokémon battle.
After seeing Hareta's Empoleon, Koya decides to test Hareta's strength and agrees to battle him. Using his Absol, Koya easily strikes down Empoleon and states that Hareta isn't worth battling. When he is about to leave, Hareta reveals that his Empoleon is still able to battle and launches a counterattack. The battle is cut short when Mitsumi freezes Hareta's attack and apologizes to Koya for Hareta's actions.
In the first round, Koya faces Jun and easily defeats all of his Pokémon in less than a minute. When Jun goes to thank him for the match, Koya coldly tells Jun that he is unfit to be a Pokémon Trainer. Hareta stands up for Jun and claims that he will beat Koya. In the next round, Koya faces another Trainer and easily defeats him with just his Metagross.
In the semi-final round, Koya is paired up against Hareta in the A-Block section of the tournament. The two Trainers battle fiercely and they both express their beliefs on Pokémon training to the other. When Hareta calls their battle fun, Koya becomes enraged and remembers back to when he was a Pokémon Trainer just like Hareta. He recalls the moment where his Growlithe was horribly defeated in a battle and had its spirit broken, preventing it from battling.
Koya calls Hareta unfit to be a Pokémon Trainer and furiously attacks with the intent of beating his beliefs into Hareta. The battle rages on until they are both down to one Pokémon. The battle is cut short when Looker informs him that Kaisei, Hareta's father, is at the tournament and so is Team Galactic. When Team Galactic begins their attack and reveals their plan to blow up the stadium with a super-powerful bomb, Koya and everyone else uses their Pokémon to have the bomb explode inside of a powerful shield, preventing it from hurting anyone. After being thanked by Hareta for his help, Koya tells him that they will settle their fight later and goes off to chase the runaway Kaisei.
Koya appears at Stark Mountain to arrest Charon along with any other Team Galactic members there despite Hareta's pleas to not arrest Mars. After Koya refuses, Hareta angrily attacks, causing Charon to escape and freeing the Pokémon that Charon was looking for, Heatran. Heatran begins attacking everyone with its powerful flames and traps them in the volcano. However, with teamwork, they manage to calm down Heatran, making it friendly and causing it to free them. They find Looker there and he asks Koya if he saw any more members of Team Galactic inside. Koya lies, saying he didn't find any more members, allowing Mars to escape because she saved him from some falling boulders earlier.
Koya later appears at the Galactic Veilstone Building to prevent Hareta and his friends from entering the building as Looker and his men are already infiltrating it. Unfortunately, the mission turns out to be a failure when Looker and his men emerge from the building after being defeated by Charon. Koya and Hareta become infuriated and team up to face Charon and his powerful army of Pokémon.
Koya and Hareta enter the Neo Team Galactic building, finding that the Pokémon were being controlled by a special machine. They team up and manage to destroy the machine and turn the Pokémon back to normal without seriously harming them. As they trek through the building, they meet Charon face to face in a special arena.
Koya is shocked to find that his beloved Growlithe is there with Charon. Charon explains that he is controlling Growlithe with a smaller version of the machine that he used on his army, making it harder to get off. Koya refuses to harm Growlithe and even jumps in front of attacks meant for Hareta. Hareta realizes that Growlithe still cares for Koya and manages to free it from the machine with the same attack that traumatized it in the first place. Finally freed from its control, Hareta explains that Growlithe never hated Koya and was willing to overcome its fear of water for him. Reunited with his beloved partner, Koya and Growlithe embrace each other.
After Charon leaves with Kaisei to try to capture Giratina, Koya and Hareta follow them using Kaisei's Plusle. With the help of Jun, they enter the Distortion World only to find Kaisei unconscious and Giratina wildly attacking everything. After finding Charon and having the situation explained to them, Hareta decides to battle Giratina. Hareta, along with Jun and Koya, battle with the legendary Pokémon, but it proves to be too strong for them. Eventually, Giratina uses its powers to pull all of them back into the real world, where it faces them in its Altered Forme.
They are saved by Dialga and Palkia, who arrive to battle Giratina. As they battle, Professor Rowan, Mitsumi, Cyrus, Mars, Jupiter, Saturn, and B-2 all come to help them. When Mitsumi hands Hareta some Berries to heal his Pokémon, everyone is shocked to find him feeding Giratina one as well. After yelling at Hareta for his actions, Hareta explains that he wants to learn about Giratina through battling with it. Giratina accepts Hareta's offer and everyone faces Hareta and Giratina in battle; once all is said and done, Giratina, Dialga, and Palkia leaves while everyone says goodbye. Sometime later, Koya and Hareta prepare to finish their battle at the Sinnoh Pokémon League Tournament.
After the tournament had ended, Koya visits Hareta at his home on Route 201. Koya gives Hareta the offer to join the International Police and take down the new evil organizations that have sprung up in other regions. Before Hareta can answer, he runs off and runs over Mitsumi. When Mitsumi talks to Koya about the offer he gave to Hareta, she explains that he is not the kind of person to join them. Koya accepts this and joins Hareta and Mitsumi as they set off onto another adventure.
This listing is of Koya's known Pokémon:
| Growlithe first appeared in a flashback. At first, Koya did not care about winning or losing, until Growlithe was brutally defeated by a Gyarados which changed Koya's attitude of battling forever. After that, he left Growlithe at a Pokémon Center and went to join the International Police. When Koya and Hareta were dealing with the Neo Team Galactic situation, they found Growlithe being controlled by Charon. Koya got it back when Hareta used Empoleon's Surf to remove the machine controlling Growlithe.
Growlithe's only known move is Endure.
|Debut||Hearts and Spirits Collide|
| Before the match between Hareta and Koya, Hareta decides to pet his Absol. Koya then demands him not to. Absol may be Koya's main Pokémon as it was the only Pokémon (other than Growlithe) that was used outside of the Pokémon League and was used more often.
In Hareta's Heart's Desire, Absol was the only Pokémon Koya used while battling Jun as it simultaneously defeated his Staraptor and Torterra without getting a scratch from Jun's Pokémon. Absol's next opponent was Dragonite. It dodged Dragonite's Hyper Beam and defeats the Dragon Pokémon with an Iron Tail.
Absol was used again when Hareta's Luxio was out on the field. It almost defeated Luxio when Luxio was covered in the rubble completely from Absol's attack. Doing so resulted in Luxio evolving into Luxray. Luxray then defeated Absol.
|Debut||A New Battle Begins!|
| Metagross first appeared when Koya sent it out to stop a Focus Blast used by Mitsumi's Infernape as the Focus Blast was heading for where Hareta and Koya are as Cynthia's Garchomp dodged the attack. Metagross was absent during the battle between Koya and Jun as his Absol defeated all of Jun's Pokémon.
Later, Metagross was sent out against a Trainer's Ambipom. Ambipom's Trainer ended up having a hard time with Metagross as none of Ambipom's attacks were able to render it useless. In the end, Metagross takes it out with Hammer Arm.
Koya sent it out to battle Hareta's Regigigas and lost. Metagross was then used to protect everyone from the bomb with Light Screen.
|Debut||A New Battle Begins!|
| Mamoswine was used in the battle against Hareta. It was proven to be very powerful as it took out Hareta's Regigigas in one hit despite Regigigas being a legendary Pokémon.
It was then used against Hareta's Empoleon after Koya's Metagross got defeated by his Regigigas. Mamoswine used its speed to try to get the best of Empoleon using its Snow Cloak Ability after using Hail. However, this was not enough as Hareta's quick planning took advantage of Mamoswine.
|Debut||Hearts and Spirits Collide|
- Eight unknown badges (prior to A New Battle Begins!)
Koya has competed in the following Pokémon Leagues:
|Japanese||コウヤ Kōya||Possibly from kō, the on-yomi reading for 光 (light).|
|English||Koya||Same as Japanese name.|
|Chinese (Taiwan)||小康 Xiǎo Kāng||From 康 kō (healthy/peaceful).|
|Pokémon Diamond and Pearl Adventure! characters|
|Hareta • Mitsumi • Jun • B-2 • Koya • Kaisei|
|This article is part of both Project Manga and Project CharacterDex, Bulbapedia projects that, together, aim to write comprehensive articles on the Pokémon Manga and CharacterDex, respectively.|