Mallow first appeared in Alola to New Adventure! while riding Tauros with her classmates Lana and Sophocles at the Pokémon School. She accidentally ran over Ash when he stumbled upon the school and, mistaking him for a new student, introduced him as such to her principal, Samson Oak. After the misunderstanding is cleared up, she decided to show Ash around the campus, showing him to her teacher, Professor Kukui, along the way. She then witnessed Ash join Kiawe in a scuffle against some Team Skull Grunts, and ws amazed by his and Pikachu's battle skills. Shortly after the battle, Ash saw the guardian deity Tapu Koko in the sky, but it flew off before Mallow and the other students could see it.
In The Guardian's Challenge!, once Ash formally enrolled in the school, Mallow saw the new Z-Ring given to him by Tapu Koko, and concluded that Ash had somehow gained the guardian's favor to receive it. The next day, Mallow threw Ash a surprise welcome party, acting as referee in a series of games between him and the other students. During a lunch break, Mallow and the others saw Tapu Koko for the first time when it challenged Ash and Pikachu's worthiness of the Z-Ring.
Mallow is an upbeat and friendly girl who gets along well with her classmates at the Pokémon School. She admits herself to be so energetic that she sometimes misses important details and jumps to the wrong conclusions. She is shown to have a firm sense of right and wrong, such as her distaste for Team Skull's bullying of other students. Much like Brock, Cilan, and Clemont, she is also a talented cook for both people and Pokémon, and is a self-proclaimed poster girl at her family's restaurant.