Doppelgänger

A is a version of oneself that exists in Adolla. For every person in the world, a doppelgänger exists. Spontaneous Human Combustion is caused by their existence intruding and merging with their counterpart.

Overview
Doppelgängers are tied to the collective unconscious of mankind. The thoughts and perceptions of people create a reflection of a person. This reflection possesses the abilities of the original but their personalities can be wildly different, as the doppelgänger is based on what the common idea of who a person is. If an innocent person is believed to be violent and cruel, their doppelgänger will have a violent and uncaring personality.

Doppelgängers manifest in the real world through different means. Haumea is able to use her abilities to pull a Doppelgänger from Adolla and bring it into the world. Shinra's doppelgänger appeared to possess the original's body. Meanwhile Amaterasu's doppelgänger has her own body, identical to the original but completely unaware of her otherworldly origins. It is currently unclear if doppelgängers have an established goal or if individuals have their own motives.

As the world heads towards a Second Great Disaster doppelgängers start to appear more human, to the point of wearing clothing and being able to talk and express themselves with ease. Doppelgängers of people who are already dead begin to appear based on people's memories of them. These new doppelgängers appear to possess the memories of the original and use them to their advantage.

Examples
Sumire conducted a doppelgänger experiment in order to call forth doppelgängers, which involved feeding Bugs to the sisters of St. Raffles Convent, leading to them becoming Infernals. She is able to summon the doppelgängers of the children killed in the Convent fire. Amaterasu's doppelgänger would appear in Year 180 and be named Sister Iris. While the First Pillar herself is full of fury and rage, the public's belief that she is a source of kindness and mercy shapes Iris into an inherently kind person.

In Year 194, When Konro Sagamiya encountered his doppelgänger, it was a Demon that copied his movements, which Hibana viewed as an act of killing the real body to impersonate it.

In Year 198, Haumea began summoning doppelgängers from the Adolla through a painful ritual. Leonard Burns' double manifests shortly after the battle at Fuchū Prison ends. It attacks the original and drags him back into its realm.

When Shinra and Inca experience visions of the world before the Great Cataclysm, Shinra is left barely holding to sanity. While his mind is elsewhere, his doppelgänger takes possession of him. The doppelgänger worked as a part of the Fire Force but when Iris approached him, he recognised her as another doppelgänger and attacked her, telling her to be quiet about what he is.

Trivia

 * There is a urban legend about doppelgängers that says there exist in the world someone who looks exactly like another person, and if the two were to meet, one of them will die.