As an Azoth Kingdom councillor, Alva is knowledgeable in the art of arcane science and the history of the kingdom. For several years, he educated Raleigh, the young prince of the kingdom, who also had interests in these subjects. However, unbeknownst to many — save for Kimia, Raleigh's sister, who did not trust him and was suspicious of his motives — he is actually a corrupt man plotting to seize control of the Azoth Kingdom for himself. For this purpose, Alva brainwashed Raleigh into swearing loyalty only to him, and developed his own type of arcane science — dubbed "neo-arcane science" — in order to create a technique called "Mega Wave", which, unlike Mega Evolution, can force many Pokémon to Mega Evolve at once and follow the orders of those with Mega Wave devices.
Alva first appeared when his henchmen, Levi and Cherie, delivered Magearna to him and Raleigh after fighting off an attack by Volcanion. While Raleigh introduced himself to Magearna, he waited quietly until Volcanion returned with Ash in tow. Using the Mega Wave jewel set in his staff, he attacked them with a Mega Alakazam, Mega Glalie, and Shiny Mega Gengar. After a short standoff, Volcanion escaped with both Ash and Magearna, angering him. Shortly afterward, Levi, Cherie, and the Pokémon caught Team Rocket sneaking into the castle. Alva met with Team Rocket in his office and, sensing an opportunity after realizing they know Ash well, recruited them to aid his cause. He supplied them with Mega Wave bracelets, along with a Heracross and Pinsir, then tracked the location of Ash, his friends, Volcanion and Magearna so he could send Team Rocket after them.
Later, after Team Rocket failed their mission, Alva personally led a massive attack against the Nebel Plateau, Magearna and Volcanion's home. He trapped Ash, Volcanion, and many of the resident Pokémon in machines that tortured them with electrical currents. When Levi and Cherie demanded that Magearna surrender to them in exchange for the safety of the hostages, it gave in and boarded their airship. Raleigh became excited by their success, but Alva simply had his Gengar paralyze Magearna and open the hatch on its chest, revealing its Soul-Heart. Realizing that Alva intended to remove the Soul-Heart, which would effectively "kill" Magearna, Raleigh tried to stop him from doing so. Undeterred, Alva had Levi force Raleigh to watch as he removed the Soul-Heart. He then discarded Magearna's body and ordered his army of Pokémon to use Hyper Beam on the Nebel Plateau, seemingly killing all of the humans and Pokémon there. Finally, he put the horrified Raleigh to sleep with Gengar's Hypnosis. En route back to the Azoth Kingdom, Meowth realized he could still hear Magearna's voice in the Soul-Heart, rallying Jessie and James to try and take it back from Alva. However, Alva blasted off Jessie and James, but held Meowth hostage so he could continue translating Magearna's voice.
Alva installed the Soul-Heart in a machine deep within the kingdom's castle, transforming part of the city's walls into a flying fortress designed by Nikola to defend the city in ancient times. When he realized that his enemies survived his attack at the Nebel Plateau and are approaching aboard Kimia's flying machine, he became excited to test the weapons the castle has on them. He began charging the fortress's laser cannon, but it did not fire, as Magearna was actively fighting his control. Alva fortified himself inside the control room while Levi and Cherie went out to fight off the group with their army of Mega Wave Pokémon. However, Ash, Greninja, Kimia, her Mega Gardevoir, and Volcanion managed to break through their defenses and reach him. He trapped them in the same electrical torture machines he used earlier, then decided to break Magearna's spirit by using Gengar's Mean Look on it, forcing it to use the laser cannon on the Nebel Plateau.
The act of destroying its home caused Magearna's soul to fade from the Soul-Heart, pleasing Alva, who then ordered another shot to finish off both the plateau and all its residents. It fired, but was blocked through a combination of the efforts of the Plateau's Pokémon and Squishy, who briefly took on its Complete Forme to overpower the attack. Volcanion then broke itself and the others free from the machines. Panicking, Alva ordered Gengar to attack, but Pikachu countered its Shadow Ball with Electro Ball. Alva dropped his staff as a result of the blowback, and Ash jammed it into the machinery, breaking the Mega Wave jewel in the process. As a result, all of the Mega Wave Pokémon reverted back to their original states and escaped from the fortress. Realizing that his plans were thwarted as a result of Ash removing the Soul-Heart, Alva locked the fortress into a collision course with the Nebel Plateau in the hopes of destroying it and killing all of his enemies. Then, he attempted to escape using a jetpack, but Greninja, who had become Ash-Greninja, threw a Water Shuriken at him. The attack failed to achieve a direct hit, but it still disabled his jetpack, causing him to spiral out of control into the forest below.
During the film's ending credits, Kimia found Alva stuck in a tree and arrested him using the same magnetic bands used earlier on Ash and Volcanion. He attempted to flee, but was pulled back onto her aircraft by the magnets.
Alva is purely evil with no redeeming characteristics. He is sadistic and megalomaniacal, and shows little regard for any lives other than his own, be they human or Pokémon. He spent years teaching Raleigh to be loyal to him, and recruited Team Rocket for their help against Ash, only to drop them without a second thought once he had no more use for them. His sadistic traits even extend to flat-out torture; his Mega Wave technique causes terrible pain to the Pokémon it is used on, and he forced Magearna to surrender by electrocuting its friends, as well as Ash and his friends' Pokémon. He later tried to force it to destroy the Nebel Plateau, for the sole purpose of breaking its spirit so it wouldn't resist any more of his commands.
When defeated, however, Alva was also shown to be cowardly and spiteful, programming his flying castle to crash into the Nebel Plateau so it would kill all of his enemies as well as Volcanion and Magearna's friends, while he attempted to escape using a jetpack.