Ogerpon and its companion after being gifted the Masks
Initially, Ogerpon and its former companion came to the land of Kitakami from a foreign land. They were shunned by the people of Kitakami due to their strange appearance, and so they lived in a nearby mountain, isolated from the villagers. Although it saddened the pair that the villagers were not welcoming, they were happy to live quietly with each other. The village mask-maker took pity on the pair and crafted four beautiful masks for them, allowing them to blend in with the people during the village festival, which made the pair overjoyed.
However, their fame attracted three greedy Pokémon: Okidogi, Munkidori, and Fezandipiti, who attacked Ogerpon's companion in their cave while Ogerpon was away, stealing all but the Teal Mask and leaving no trace of the companion when Ogerpon returned, with evidence of a struggle. Perhaps if only to search for its friend, filled with rage, Ogerpon attacked and defeated the three Pokémon in the village, but its anger caused the villagers to fear it even more. Thus, Okidogi, Munkidori, and Fezandipiti were immortalized by the unaware villagers as the Loyal Three, while Ogerpon was left alone in its cave, saddened by the events that had transpired. Although the village mask maker desperately attempted to tell the truth of the matter, nobody in the village took him seriously. In order to prevent further generations of his family from being persecuted for this truth much like he was, he passed down the true story as a family secret, which later made its way to his descendants Carmine and Kieran, two Kitakami natives.
As a result of the villagers' interpretation of the event, there is a myth that Ogerpon will supposedly steal the souls of humans without masks late at night. It resided in the Dreaded Den for many years.
The Teal Mask
Ogerpon after dropping the mask
During the Mask Festival, the player and its friend Carmine meet Ogerpon, mistaking it as a child. Ogerpon dropped its mask and ran away after being seen at the outskirts of the festival. Unbeknownst to Ogerpon, the player and Carmine fix the damaged mask with the help of her grandfather. After the Loyal Three are suddenly revived, they stole Ogerpon's three other masks (the Wellspring Mask, Hearthflame Mask, and Cornerstone Mask) and started attacking the Mask Pokémon at the Dreaded Den, who appeared defenseless. They were chased off by the player and Carmine, who returned the Teal Mask. Kieran attempted to do so initially, but presumably due to being saved by the player and meeting them initially at the festival, they only accept the mask when done so with the player.
Following this, Ogerpon now walks alongside the player wherever they go in Kitakami. Initially when brought to the village, it appears scared of entering even with the player and their friends. Ogerpon is asked to stay outside the village whilst Kieran, Carmine and the player gather clues about the Loyal Three's current whereabouts, though only does so after the player makes the suggestion themselves, much to Carmine's chagrin. After enough clues have been gathered as to Okidogi, Munkidori, and Fezandipiti's locations in Kitakami, it watches the player and Carmine's Multi Battles against the now titanic-sized members of the Loyal Three, happily posing in pictures after each mask is retrieved.
After retrieving the Cornerstone, Wellspring, and Hearthflame masks from Okidogi, Munkidori, and Fezandipiti, respectively.
After retrieving the masks, Kieran tells the player, Carmine and Ogerpon to return to Mossui Town. Once they arrived, they learned that he had told the truth to all the villagers, which led them to profusely apologize to Ogerpon, despite its initial hesitation to enter the village. The group returns to the Dreaden Den to bid farewell to Ogerpon, but it runs over to the player and implies that it wants to travel with them. This irritates Kieran, who, after having dreamed of meeting Ogerpon since he was a small child, challenges the player to a battle to win its affection, despite Carmine's reminder that it's Ogerpon's choice as to who it decides to travel with. After his defeat, the game claims that Ogerpon wants to test its strength against the player, and gives them the choice to accept. During their battle, it Terastallizes its Hearthflame Mask, Wellspring Mask, Cornerstone Mask, and Teal Mask in four consecutive battles with no interruptions, with the masks gaining power due to Ogerpon's strong memories and determination following its journey with the player. The Teal Mask in particular invokes the memories of its former companion from many years ago for a final boost in strength, before allowing itself to be captured by the player, where it entrusts them with the transformation masks.
It participates in a final battle against Carmine as per the Trainer's request, before the events of The Teal Mask come to a close.
Personality and characteristics
Ogerpon expressing joy
Ogerpon has a curious and mischievous personality. It appears to long for companionship, as evidenced by it tentatively approaching the ongoing Kitakami Festival, attempting to blend in with the villagers while wearing a mask. When its face is initially seen by the player and their friend, it's understandably shy due to being shunned by humans. After being saved by the player following the revival of the Loyal Three, it warms up to them over the events of The Teal Mask. As a result, it often mischievously ignores Carmine's orders, and only listens to the player. It's clear that Ogerpon thinks about its previous companion even after warming up to the player, with the companion being implied to have fallen at the hands of the Loyal Three. Ogerpon's experience of being seen as an outcast when initially coming to Kitakami had an impact on other characters such as Carmine, (following the reveal of its true history) who was initially cold to outsiders coming to its village.
Ogerpon has four forms, which each have their own special Terastallized State. The masks created for Ogerpon and its companion are given power by Ogerpon's intense emotions, which cause a significant change to its appearance when Terastallized, unlike most other Pokémon.