Koga is a character appearing in Pokémon Adventures, who is a member of the Elite Four in the Johto region, was the Gym Leader of Fuchsia City and was an executive of Team Rocket in the Kanto region.
Koga is identical in physical appearance to his game counterpart. His clothing varies throughout the series' medium, in which his first appearance depicted him wearing a bandanna like cloth, except for when he becomes a member of the Johto Elite Four in the HG/SS arc, with his clothing resembling that of his game counterpart's.
Koga's personality is first depicted to be cruel and merciless, as shown during his battle against Blue and Red in Pokémon Tower, as he uses Articuno to nearly freeze the pair to death. However, after their conflict, Koga returns once more to face off against the Elite Four member Agatha, teaming up with his nemesis, Blue to defeat her, and displays more admirable characteristics such as bravery and concern for others.
Red, Green & Blue arc
Koga first appears at Mt. Moon, where he is fervently searching for the Moon Stones to bring back to Team Rocket. He is stopped by an untimely appearance by Red and Misty, who thwart his operation. Koga then makes an appearance at the Pokémon Tower in Lavender Town, planning to use his Gastly to revive the deceased Pokémon corpses stored there. However, Red and Blue form an unlikely alliance against him, but experience difficulty in the battle, primarily due to his Arbok's ability to regenerate and control separated body parts. After this, Koga brings Articuno to Saffron City under Giovanni's orders, who then proceeds to combine the three legendary birds together into an amalgamation of the three. As Red and Blue oppose his plans at Silph Co., he nearly kills them by using Articuno to freeze them to death. However, Blue manages to defeat him by using his Charizard's flames to melt Articuno's ice.
Koga and the other Team Rocket executives team up with the Pokédex Holders, in order to prevent the Elite Four's plans from culminating on Cerise Island. Koga joins forces with Blue, preparing to face off against Elite Four Agatha, which proves to be a perfect match for both opponents. As they face off against each other with their Pokémon, Blue is temporarily paralysed by Haunter's Lick. In response to this, Koga throws a multitude of shuriken at the surrounding area, which Agatha believes to be a fruitless task, and that he is nearing the end. However, Koga reveals that they were Poké Balls containing a potent Paralyz Heal, which reached Blue, healing him from the ailment. The two formulate a new plan, with Koga instructing his Golbat to use Leech Life on himself, which results in Agatha's Arbok to not be able to change the pattern on its crest. As Agatha is seemingly defeated by Koga's Confusion, the two proceed to try and navigate their way through the labyrinthine pathways underneath Cerise Island.
However, Agatha was merely manipulating Koga through her Gengar. She uses her Gengar to manifest as their shadows, proceeding to drain the life from Koga and Blue. However, they saw through her trickery and defeated her, just as Cerise Island began to collapse. As Blue supports him through the journey, Koga uses his Muk to protect his body. After the Cerise Island incident ends, he leaves Janine in charge of the Fuchsia Gym, promptly disappearing for a short period of time. He regroups with three other trainers, all of whom have been abandoned by their respective criminal groups, Will, Bruno and Karen. Bruno then trains Koga during his absence after saving him from being killed on the island.
HeartGold & SoulSilver arc
Koga reappears alongside his teammates at Johto's Pokéathlon Dome, announcing the reformation of the Johto Elite Four.
Rhydon was first introduced as its pre-evolution, before Koga injected it with a serum which induced its evolution. In this form, it was powerful enough to defeat both Red and Misty easily.
Grimer was used by Koga as an outfit, as well as being used to constrict his opponents. He uses it to the extent that Red is unable to call upon his Pokémon for assistance, and made him grievously close to suffocating.
Articuno was captured by Koga for Giovanni's plans, and he used it to primarily defeat Red and Blue, shortly before it fused with the two other birds. However, once Koga was defeated, it was returned to the wild, before being captured by Green.