Arceus is one of the main antagonists of the HeartGold & SoulSilver arc, planning on destroying humanity due to its mistrust of humans. It was the target of Team Rocket's plans to create Dialga, Palkia, and Giratina.
Arceus first appeared in a flashforward in Out-Odding Oddish, battling against Gold. It made its first actual appearance in With a Little Help From Hitmonchan, having been awakened by Team Rocket at the Hall of Origin and having come to the Johto region due to all of its Plates being currently located there. Crystal decided to try catching it, but Arceus proved to be too strong and escaped. Crystal tried to pursue it, and was soon blasted out of the sky.
Soon after, Arceus appeared at the Ecruteak Gym and promptly destroyed the building. Gold attempted to battle it, but Arceus easily pushed away Exbo's Blast Burn before flying away to the Ruins of Alph. Upon arriving at the ruins, Arceus formed a barrier around itself and Gold, forcing him to battle it. Soon Silver appeared at the site too, having collected all 16 Plates. Sensing them, Arceus transported everyone in the area to the Sinjoh Ruins. Upon arriving, Team Rocket captured Arceus and forced it to the Mystri Stage. Archer the used the Plates to take control Arceus and forced it to create Dialga, Palkia, and Giratina.
The Pokédex holders attempt to stop Arceus's creation process, but even though it momentarily seemed like they had succeeded in this, Arceus finished creating the Legendary Pokémon as soon as it was done absorbing the Plates. Lance revealed that Arceus had been intending to create the Legendary Pokémon regardless as a test to see if humans could stop their fight and save the world. Gold attempted to help Arceus regain its faith in humanity, and eventually succeeded after bonding with his Togebo. Satisfied with Gold's connection with his Pokémon, Arceus stopped the battle between the Legendary Pokémon and transported everyone back to the Ruins of Alph before taking its leave.
Arceus once lived in harmony with humans, but was betrayed when humanity stole Arceus's power and converted it into the Plates. Due to this, it grew to mistrust humanity. It began to exhibit strong hostility towards them, sometimes attacking them on sight unprovoked. Despite this, Arceus wished to see the good in humans again, waiting for one to show it their heart and end its suffering. After seeing Gold bond with his Togekiss, Arceus begun to regain some of its trust in humanity.
Arceus is a very powerful Pokémon, managing to defeat the Pokédex holders of Johto without much effort. Its signature Ability, Multitype, allows it to become any type it wishes using the Plates, consequently changing the type of its signature move, Judgment. When it is at the Mystri Stage in the Sinjoh Ruins, it is able to summon a swarm of Unown and create Dialga, Palkia, and Giratina, emphasizing its status as the Alpha Pokémon.
