Prior to the start of the Sun, Moon, Ultra Sun & Ultra Moon chapter Mimikyu disguised himself as a Pikachu to try and be accepted by others. Everyone continued to treat him poorly though and he grew resentful and settled down at the Abandoned Thrifty Megamart and became the Totem Pokémon of the place.
Penny first appeared in A Photoshoot and the Abandoned Thrifty Megamart at the Abandoned Thrifty Megamart, being mistaken by Sun as a doll. Calling him ugly Mimikyu destroys Sun's camera and attacks him. Sun sends out Dollar to attack him and figures out he's the Totem Pokémon he needs to take a picture of. Using the Rotom Dex as his new camera Sun attempts to take a picture but the Disguise Pokémon proves to be too fast. Sun and Dollar use their Z-Move Inferno Overdrive to try and slow him down but Mimikyu's Ability Disguise blocks all of the damage. Mimikyu continues to attack Sun instead of Dollar which prompts him to figure out whats angering him. Rotom interprets his thoughts and Sun discovers he's angry at him for dissing his appearance. Deciding the best way to help Mimikyu is to get him outside Sun offers him a spot on his team, which Mimikyu accepts. The two then take a photo together to complete Sun's trial.
In Truth and the Mastermind Behind Team Skull, Penny, along with the rest of Sun's team, helped to rescue Moon from an attacking Xurkitree. By combining their attacks, they managed to knock Moon from Xurkitree's grasp, though Xurkitree immediately retaliated by smacking them away onto a nearby car.
In Darkness!! The Mysterious Claws of Necrozma!, Penny was used alongside the rest of Sun and Moon's teams to assist Lunala against Necrozma, but was stopped by Lusamine's Naganadel.
After returning to Alola, Sun finds Necrozma, who wants to absorb the light energy from the region. Sun sends out his team to attack, who get thrown back by the Prism Pokémon. This causes Dollar to evolve into Incineroar, who manages to push it off a cliff. The group goes after it on top of Drachma, though it easily pushes them back, save for Drachma, who is able to use its large body to hold Necrozma down.