Professor Kukui debuted in Alola to New Adventure!. He was first seen entering his classroom, where he was introduced to Ash. He later watched as Ash battled Team Skull and was impressed by his battling skills. After Team Skull's defeat, he told Ash about Z-Moves and the island challenge. Following Ash's decision to attend the Pokémon School, he allowed Ash to live with him during his stay in Alola. At the end of the next episode, he handed Ash a new Pokédex, which then got inhabited by a Rotom.
In Rocking Clawmark Hill!, he and Ash returned home one night, only to notice the absence of Rockruff. Rockruff soon returned injured, prompting Kukui to heal it with a Potion. Upon further investigation, they found out that it had been training on Clawmark Hill. After watching Ash train Rockruff, Kukui noticed the bond they had developed and encouraged Ash to catch Rockruff.
In Alola, Kanto!, Kukui accompanied his students and Samson Oak to Kanto for an extracurricular activity, where they visited Professor Oak's Laboratory. In the following episode, he and the others traveled to the Cerulean Gym, where his students battled Misty and Brock. Seeing these battles inspired him to establish an official Pokémon League in Alola.
In A Dream Encounter!, Kukui told Ash about Professor Burnet and her research on Ultra Wormholes. He later introduced her to him when she, along with the president of the Aether Foundation Lusamine, Faba, and Wicke, visited his home to meet Ash and Nebby. During his class's sleepover in A Dream Encounter!, Kukui began to bond with Burnet, who also stayed over at his house.
In Family Determination!, Kukui accompanied his students to the Altar of the Sunne on Poni Island to catch up with Lillie, who had ran away from home with her brother, Gladion, in search of a way to save Lusamine, who had been abducted by a Nihilego. Later, while watching Alola's guardian deities bless Nebby, he received a phone call for help from Professor Burnet and left to help her out. He soon witnessed Burnet and her Munchlax being surrounded by a group of Jangmo-o and Hakamo-o, but using his Braviary he was able to drive the Scaly Pokémon off. By the time the two professors returned to the altar though, Ash and the others had already ridden off on Nebby into an Ultra Wormhole. Kukui and Burnet stayed behind and waited until the group finally returned with Lusamine.
Professor Kukui and Professor Burnet about to get married
In The Professors' New Adventure!, during a walk on the beach with Burnet, Kukui proposed to her. A few days later, the wedding ceremony, which was arranged by his students, was held on the beach outside his house. Since their marriage, Burnet has moved in with Kukui and Ash.
Kukui's alter-ego, the Masked Royal, debuted in Pushing the Fiery Envelope!. When Ash and his classmates visited the Battle Royal Dome, Ash's Litten got agitated upon seeing Royal's Incineroar. This prompted him to invite Ash, along with Kiawe and Sophocles, to take part in a Battle Royal, which the Masked Royal easily won. Days later, after seeing Litten training to master Flame Charge, Kukui, dressed up again as the Masked Royal, showed up to have a one-on-one battle with Ash. During the battle, Litten evolved into Torracat and was able to perfect Flame Charge. Despite this, however, the battle ended in the Masked Royal's victory. Afterwards, the Masked Royal declared that he and Incineroar were now Torracat's rivals.
The Masked Royal reappeared in A Young Royal Flame Ignites!, where he faced a member of the Revengers, Mr. Electric, and his Electivire in a Battle Royal. However, due to interference Mad Magmar's Magmortar, Mr. Electric was disqualified. The Masked Royal then got challenged by Mad Magmar to a battle for the title of the strongest fighter, which he accepted. The next day, Kukui accompanied Burnet and his students to the Battle Royal Dome, but began feeling nervous as he had to take on his role as the Masked Royal without his identity being found out. After managing to sneak away, he was able to reach the ring in time, where his battle would begin. However, Mad Magmar played dirty again by having other members of the Revengers gang up on Incineroar until Ash's Torracat interfered. Teaming up with Torracat and Ash, the Masked Royal and Incineroar took on Mad Magmar and Mr. Electric and eventually managed to beat them, despite all the interference.
In Filling the Light with Darkness!, mysterious dark clouds appeared over Alola, leaving Kukui and all other adults tired and without motivation. Later, after this was resolved, Kukui met up the Ultra Guardians, Burnet, Lusamine, Faba, and Wicke inside the Ultra Guardians base, where he oversaw Lunala recovering. Later, when the Ultra Guardians left for Ultra Space with Lunala, Kukui and Faba volunteered to aid them. On Ten Carat Hill, he and Faba confronted Team Rocket's Matori Matrix by using their Pokémon to damage the team's airship and prevent them from entering the Ultra Wormhole. Afterwards, Kukui and Faba faced Gozu and his Mega Aggron in battle, but their Pokémon were swiftly beaten. In the next episode, Kukui asked Faba if he had more Pokémon, and the latter replied he did not. Thus, Kukui was forced to send out his Incineroar and perform its Z-Move, Malicious Moonsault, which managed to defeat Mega Aggron. However, this also caused him to reveal his secret identity to Faba, who in turn revealed that he was a big fan of the Masked Royal. Later, alongside Incineroar and Faba, Kukui joined the rest of Alola in showering Necrozma with Z-Power so it could return to its true form. After Necrozma returned to its Ultra form, Kukui watched as the Ultra Guardians, Nebby, and Lunala returned to Alola, alongside Faba, Burnet, Lusamine, and Wicke.
In Turning the Other Mask!, the Masked Royal announced a handshake event to be held on Melemele Island. When Ash and Burnet told Kukui they wanted him to go with them to the event, Kukui asked Molayne to help keep his Masked Royal identity a secret. The day of the event, Molayne prepared to change into the Masked Royal costume, only to be tied up by Faba and James, who had mistaken him for an imposter. When Mad Magmar came around looking for revenge, Faba donned the Masked Royal costume and tried to defeat him with Incineroar, only to be easily defeated. After freeing himself, Molayne had his Magneton shut the lights off, giving Kukui the time to slip back into his Masked Royal costume and defeat Mad Magmar.
Ash almost discovering the Masked Royal's true identity
Professor Kukui is a fairly serious, but happy character. Despite this, he usually likes to add the names of moves into conversations every now and then, much like how Samson Oak uses Pokémon puns. He has been shown to be also break from his serious and calm attitude, as shown in Crystal-Clear Sleuthing!, when he was completely shocked at the notion that Kiawe had stolen Ash's missing Electrium Z. Like Rotom Pokédex, he enjoys watching the television show Alolan Detective Laki and try to figure out who the culprit is during the episode. Much like his use of move names when he talks, he was shown to find it funny that the viewers assumed that the Rockruff which he befriended was actually his. The fact that Professor Kukui befriended the stray Rockruff, when he came upon the Pokémon, who had recently been attacked with Teeter Dance and decided to nurture it, shows that he is also a friendly and hospitable person.
Professor Kukui is a skilled Pokémon Trainer, and as the Masked Royal, he is able to handle battling multiple Pokémon at once in Battle Royals. The Masked Royal is the Battle Royal Dome's star Trainer, having a known record win streak of at least 21 victories. Despite some comically close calls, no one has been able to discern the Masked Royal's true identity. The only exceptions are Molayne; Faba, who realized the truth after Kukui inadvertently blew his cover; and Ash's Torracat, who was able to recognize him from his scent.