Volcanion was initially distrustful of humans because it was aware of the harm that they did to other Pokémon. Eventually, Volcanion begins to trust humans after seeing Ash and his friends protect the Pokémon of Nebel Plateau from Alva. It is protective of Magearna, having met it when Magearna went off and got lost. Volcanion apparently seems to be allergic to the flowers it carries, even when fainted.
500 years ago, Magearna, an Artificial Pokémon, got lost. Volcanion noted that Magearna was a mechanical Pokémon, but befriended it anyways. As time passed, Volcanion took care of Pokémon that were abused by humans, eventually viewing humans as those who would harm innocent Pokémon.
Volcanion protects Magearna throughout the plot of Volcanion and the Mechanical Marvel. A mysterious device latched it to Ash. Volcanion was annoyed by the fact that he have a human accompanying him. Volcanion unintentionally hurt Ash multiple times without much regard for him. When confronted by Alva and his associates, Volcanion used Steam Eruption to throw them off the group's track. When they arrive at Nebel Plateau the group attempt to befriend the Pokémon there, but find that they are scared of them. Volcanion tells them that the Pokémon here were those that it brought in after watching them get harmed by humans.
When Magearna gets captured, Volcanion is concerned for its safety and accompanies Ash and his group to try and save it. Alva manages to extract Magearna's "Soul Heart" and uses it to power an ancient weapon. Volcanion is saddened with Magerna's apparent death but the weapon is collapsing after stopping Alva. As the group tries to escape, Volcanion stays behind to ensure the weapon would not crash into Nebel Plateau. While Volcanion managed to destroy the weapon, it was exhausted and apparently seemed to have died in the crash. Magearna, who was returned to normal, brought up some flowers and exposed the pollen to Volcanion, causing it to revive and tell Magearna that it did not like having the pollen up its nose.
With situation defused, Volcanion tells Ash and his group that they are honorary members of Nebel Plateau and that they can visit anytime they wanted.