From the games explanation:
A long, long time ago, a man and an ogre came to Kitakami from a
foreign land. The people of the village feared the man and the ogre,
who looked so much different from them... And so they refused to let
the travelers come anywhere near the village. The man and the ogre
were saddened that the villagers did not welcome them... But they were
happy just to have each other. They settled quietly in a cave on a
mountain. There was only one villager that pitied the two travelers:
The village mask maker. He made several masks for the man and the
ogre. The masks were brilliant works, adorned with gems that the man
had brought from somewhere far away. By wearing these masks, the
travelers could hide their true faces and mingle with the villagers.
The man and the ogre were overjoyed. They thanked the mask maker for
his kindness. Wearing the masks, the man and the ogre started secretly
joining the village festival. The mysterious pair soon became the talk
of the village because of their brilliant masks. In fact, rumors about
them quickly spread far and wide, even to distant lands. But rumors of
exquisite, shining masks attract not just innocent curiosity... A
group of greedy Pokémon soon made their way to Kitakami. These Pokémon
sneaked into the cave in which the man and the ogre lived... and tried
to make off with the masks, which were very carefully stored away. The
man happened to be there. He managed to hold on to one of the masks...
but he was not strong enough to protect them all. The Pokémon stole
the other three masks. Several hours later when the ogre returned to
the cave... it found its beloved in ruin. All that was left were signs
of a struggle and a teal mask. The ogre donned that mask and went down
to the village, perhaps to search for its friend. It found the greedy
Pokémon there, gloating over their stolen masks, and defeated them.
The villagers, of course, had no idea what was happening nor why. All
they saw was the raging ogre. And in great fear, they fled. The
villagers thought the three Pokémon had fallen trying to protect the
village from the ogre. To honor their "sacrifice", they named them the
Loyal Three and interred them with care. Wounded and weak, the ogre
returned to its cave--alone and with great sadness.
TLDR; A man and an ogre (Ogrepon) went to Kitakami. They went to the village, but the villagers were very scared of them because they were very different from the villagers. They went to a cave and called it home. There was a mask maker, and he pitied the travelers, so he made masks for them to be able to go in the village and mingle with them. People made rumors about these people, and soon other lands heard about these rumors, leading to the so-called "Loyal Three" to try and steal the masks. The man fought the Pokémon but died, leaving only one mask for Ogrepon to use. Ogrepon was very sad and mad so she went to the village and fought the Loyal Three. The Loyal Three fainted, but then the people thought they died. They also thought the ogre was the bad guy, and the Loyal Three the good guys. So, then they made a festival just to celebrate them. The ogre was very sad and went to its cave, living alone.
It's implied that they were revived since Kieran had the Teal Mask when he punched the shrine pole, and the Teal Mask is known for healing and life-giving/restoring properties.
This is probably true.
Source (Carmine / Kieran's grandpa in the game)
