After the Cataclysm and the encounter with the Unknown God, the sibling woke up first and traveled Teyvat with Dainsleif.
However, before they could reach the end, the sibling discovered the truth of this world and parted ways with Dainsleif.
They then encounter the sinner of Khaenri'ah and they start to work with them and the abyss order.
When the traveler encounters their sibling in front of the Defiled Statue, the sibling stops them from attacking the Abyss Herald.
After a brief confrontation, the sibling tells the traveler not to stop them or the abyss. The traveler attempts to convince them to
leave Teyvat and journey together again, however the sibling declines, saying that they cannot continue their travels until their war with
the divine is over. The sibling then informs the traveler to continue their journey and discover the truth of this world themselves.
In the Sumeru archon quest, Nahida informs the traveler that Irminsul labels them as a Descender (an entity that is suggested to transcend the laws of Teyvat), the fourth one to be exact. However, the sibling is not labeled as such, which implies that Irminsul contains records about the sibling which is impossible unless the sibling is from Teyvat. Nahida also notes that the sibling's adventures through Khaenri'ah and Teyvat are present, but the records become "fuzzy" near the end. As such, Nahida deduces that there is likely someone that is purposefully hiding that information.
A common theme seen throughout the game is the theme of twins. Almost all of the Archons that are portrayed have some kind of twin, with Venti taking the form of his dead friend, Ei being the twin sister of Makoto,
Nahida being another version of the Greater Lord, and Furina being the humanity of Focalors. Twins are a constant theme. However, in all the twins that have been shown, only one of the twins have survived.
As such, combining that theme as well as the fact that the sibling is considered to be from Teyvat, many people have come to speculate that the Abyss sibling seen in the game is in fact a clone of the Traveler's real
twin. Some speculate that the twin was created through Kheanri'ahn technology, whether it was to obtain other-worldly powers or to revive the sibling should they have died during their time in Khaenri'ah, no one is really sure.
Fun fact: The Abyss sibling's title within the Abyss order is princess/prince, suggesting that there may be a "king/queen" or higher ranking above the sibling.
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