Spoiler warning: this article may contain major plot or ending details.
Teal Mask
Kieran is a student of Blueberry Academy native to Kitakami. He and Carmine are chosen to accompany the incoming students from Naranja AcademyS/Uva AcademyV on a cultural tour of the area under the watch of Carmine's teacher Briar. Kieran is paired up with the player character, and accompanies them around Kitakami as they search for three signposts that tell the story of how the Loyal Three laid down their lives to protect the land from the evil Ogerpon. During this time, Kieran admits that he actually admires Ogerpon for her strength and independence.
At the end of the first day in Kitakami, the player, Carmine, and Kieran attend the Festival of Masks. The player and Carmine have a chance encounter with Ogerpon, who flees and drops her Teal Mask. Carmine decides to keep this incident from Kieran, fearing that the knowledge he missed a chance to meet his idol would break his heart. The next day, the player and Carmine learn from the siblings' grandfather that the "Loyal" Three were actually a group of bandits who attacked Ogerpon's Trainer and stole three masks made for her by the siblings' distant ancestor; Ogerpon slew the Lousy Three, but the villagers believed she was attacking them and came to fear her. Unbeknownst to them, Kieran overhears their conversation and concludes that they are excluding him out of contempt. While the player and Carmine search for materials to repair the Teal Mask, Kieran steals the mask, accuses them of treating him like an outcast, and demands the player defeat him in a battle to reclaim the mask.
After the player defeats Kieran, he surrenders the mask as agreed. However, the Lousy Three unexpectedly come back to life and are given food and Ogerpon's stolen masks by the ignorant villagers. While the player, Carmine, and Ogerpon pursue and defeat the Lousy Three, Kieran convinces the villagers that the old legend was mistaken and that Ogerpon was innocent all along. With Ogerpon's reputation rehabilitated, the group returns to Ogerpon's mountain home, where Ogerpon chooses the player to be her Trainer. However, a jealous Kieran demands a final battle with the player for the right to take Ogerpon, irrespective of Ogerpon's wishes. The player defeats Kieran, who watches them catch Ogerpon before fleeing home, locking himself in his room, and refusing to come out or see the player off.
After the end of the tour, Kieran and Carmine are to return early to Blueberry Academy due to an unexpected development in the Great Crater of Paldea. Kieran, now consumed by his resentment towards the player and obsession with strength, vows to become even stronger and ominously declares, "Just you wait" to the player.
Indigo Disk
Upon returning to Blueberry Academy, Kieran becomes fixated on Pokémon battling, going so far as to rise in the ranks of the school's unofficial League to become Champion. However, Carmine and the BB League's Elite Four, Drayton, Lacey, Crispin, and Amarys become concerned when he begins using his new authority to bully other students out of the club. Despite Lacey's insistence to keep this as a 'League matter' and have Drayton deal with Kieran himself, Carmine secretly lobbies to have the player brought over to the school under the auspices of wanting to show off her home turf to get them to snap some sense into her brother. Upon discovering the player's arrival at the school, Kieran, against Lacey and Amarys' objections, has the player entered into the BB League at a high stage so he can prove that he is superior to them. However, despite the amount of training he's done, the player's skill as an official League Champion proves too much for Kieran to overcome and he is defeated, causing most of the club to abandon him.
Now humiliated in front of the school, Kieran scrambles to come up with a way to get back at the player. When Briar informs them and Carmine that she's recently gotten permission from Paldea League chairwoman and Top Champion Geeta to search Area Zero for a mysterious Pokémon called Terapagos, Kieran sees this as the perfect opportunity and signs up, reasoning that catching and utilizing a Pokémon even the player doesn't know about will give him the edge needed to defeat them. However, upon reaching the furthest point in Area Zero's lowest depth, Terapagos shows more interest in the player than Kieran, forcing him to use a Master Ball he received during his time as BB League Champion to effectively steal it from them. However, when the player is still able to defeat him, Kieran, under a glory-blinded Briar's instructions, Terastallizes the Tera Pokémon and demands it battle the player again. This proves ruinous, however, as Terapagos turns on and attacks Kieran, destroying the Master Ball he caught it with and forcing the player to intervene and catch it.
The shock of Terapagos attacking him forces Kieran to finally realize the error of his ways, resigning from the BB League and requesting that he and the player start over as friends. The player presumably agrees, and they return to Blueberry Academy.
Upon defeating Kieran the third time in the League Club Room, he will trade a Applin to the player in exchange for almost any Pokémon excluding Shiny, Legendary, or Mythical Pokémon.
When Kieran punches the Loyal Three's shrine after his third battle, a purple mist can be briefly seen in his fist, implying the trio's revival may have something to do with Kieran himself. Kieran also has the same purple in his hair during The Teal Mask as the color of the Toxic Chain, further implying some sort of connection.
Kieran is the first NPC shown capturing a Pokémon with a Master Ball.
↑Carmine says that Kieran is going through "teen angst", implying that he is a teenager;[1] in other languages, Carmine simply attributes his behavior to puberty or adolescence. His actual age is not otherwise known.
↑In the French version of the game, he is explicitly 14 years old.[2]
References
↑Carmine: "What's [Kieran's] problem? He's been all bent out of shape since yesterday. [...] Must be teen angst or something. It happens!" (Pokémon Scarlet and Violet)
↑Roseille: "Kass a quatorze ans, alors il aime bien les créatures qui font peur. C’est aussi pour ça qu’il préfère le Monstre aux Adoramis." (Pokémon Écarlate et Violet)
