The unknown god is someone that the twins meet in the beginning cutscene and is the one responsible for separating them. She declares that she is the Sustainer of the Heavenly Principles, however there is no implication as to what this actually means yet. She weilds a cube-like magic and this cube magic can also be seen in visuals of the Cataclysm and the destruction of Khaenri'ah.
Paimon is the first character the traveler meets when they awaken from their slumber after the fight with the Unknown God. The traveler had fished out Paimon from the scene, saving her from near drowning. Paimon is a small, floating entity, and declares herself to be the traveler's guide. Many players have taken notice of Paimon having several motifs that can be seen in things related to Celestia, such as the statues of the seven, as well as having an upside down triquetra (a very prominent symbol within Teyvat) which can be seen as either the Trinity from Christianity, or as the cycle of lfe from Celtic or Druid ideas. Similar to the Archons, Paimon is the name of a demon from The Lesser Key of Solomon and is one of the 9 Kings of Hell. Paimon is also known to be the most loyal to Lucifer.
As stated, Paimon has several motifs that can be seen in things related to Celestia, specifically the golden accents that are seen in fully upgraded Statues of the Seven. She also share similar elements with the Unknown god, from the white hair, black cape, and star motifs. Many players have also taken notice that when opening the menu, Paimon's menu, Paimon appears and time essentially stops. As such, it's been deduced that Paimon has control over time in some way. So there are many speculations of the possibilty that Paimon will betray the traveler sometime in the future or will be related to the Unknown God at some point.
Fun fact: The Unknown God shares a design with the Honkai Impact 3rd (another Mihoyo game) character, Kiana / Herscherr of the Void.
In fact, there are many characters who are considered "expies" or copies of Honkai Impact 3rd characters.
(Ei/Beezlebub - Raiden Mei, Alhaithem - Su, Nahida - Theresa, etc)
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