Professor Kukui is the Pokémon Professor of the Alola region and a teacher at the Pokémon School on Melemele Island, where he plays a central role in the daily lives of his students. His area of research is Pokémon moves, an interest he has held since childhood, and he is known for his passion to the point of allowing himself to be struck by attacks in order to study their effects firsthand. In his youth, he took part in the island challenge alongside his childhood friend Molanye, and he later traveled to the Kanto region to compete in its Pokémon League, where he reached the Elite Four before losing to Lance. That experience stayed with him and shaped his long-held ambition to create a Pokémon League of his own in Alola, a goal he eventually achieves over the course of the series.

As a person, Kukui is energetic, warm-hearted, and always enthusiastic about research and battle. He has a relaxed, sometimes absent-minded manner and a habit of declaring that adventure is out there, yet he is deeply dedicated to his students and to the people close to him. When Ash Ketchum arrives in Alola, Kukui welcomes him into his home, provides him with a Rotom Pokédex and a Z-Ring, and encourages him to enroll at the Pokémon School. In this way he becomes much more than a teacher; he acts as a mentor and a father-like figure to Ash, supporting his growth as a trainer and pushing him to improve.

Kukui also leads a double life as the Masked Royal, a celebrated competitor in the Alola Battle Royal. Under this disguise he battles challengers with great success, and he uses the identity to test and motivate Ash without revealing his true nature. The eventual reveal of his secret adds depth to his role, showing that his love of battle and his desire to help young trainers grow are two sides of the same character.

In his personal life, Kukui is married to Professor Burnet, a researcher who supports his work and shares his home. Their relationship grows throughout the series, and in the finale they welcome a child whom Kukui names Lei in honor of Ash and the journey that shaped their time in Alola.

As a trainer, Kukui is formidable. His partner is an Incineroar that has been with him since it was a Litten and that shares many of his mannerisms; it can use the Z-Move Malicious Moonsault and is a powerful force in Battle Royal matches. He also keeps a Midnight-form Lycanroc, a Braviary, and at times calls on other Pokémon such as Lucario, Venusaur, and Empoleon for research, defense, or battle. After Ash wins the Alola Pokémon League, Kukui faces him in an exhibition match with a full team, demonstrating both his strength and his deep respect for the trainer his student has become. Through his teaching, his research, and his founding of the Alola League, Kukui leaves a lasting mark on the region and on those who trained under him.