Onashia

Onashia was a Guardian of the Spring. Heavily implied (if not confirmed by Yun) to be future version of Dominura; refer to Dominura for more information.

Profile
Onashia was a citizen of the advanced nation of Simulacrum, and a member of Chor Dextra. However, throughout her life she refused to pick a permanent sex. As punishment, she was eventually made into the Guardian of the Spring. For all eternity she was cursed to watch every citizen of Simulacrum pick their sex, and if they did not know what to pick, she was to communicate with Tempus Spatium and choose for them. Onashia was also forced to attend political meetings all around Simulacrum as a physical symbol of peace, being hailed as the human representation of Tempus Spatium despite her past 'error' of not picking a sex.

It is revealed through Onashia that those who do not choose a gender eventually turn into living, immortal dust that can only be freed and allowed to die after being embraced by another. One day, Onashia meets Yun and tells her of her plight. Since she is the Guardian of the Spring, Onashia has not been allowed to embrace another and has instead been forced to live on in her form, enduring vast amounts of pain. Yun feels immense sympathy for Onashia, not thinking it fair that she has had to suffer ceaselessly for one decision. Therefore, at the end of Simoun, Yun decides to free Onashia, embracing her and telling her she is truly beautiful. Onashia cries of joy as she fades away, finally at rest, in Yun's arms.

Afterwards, Yun becomes the new Guardian of the Spring in Onashia's place. It is unknown if Yun chose a permanent sex before becoming the Guardian.

Theory
It is heavily implied that Onashia is the future version of Dominura. As stated in behind the scenes clips, Dominura is from a different planet then the rest of the cast, and thus ages differently. She could theoretically live long enough to become Onashia. It is also said that Onashia was a member of Chor Dextra. The only people who survived Chor Dextra were said to be Dominura and Aer's grandfather. Additionally, Dominura never chose a permanent sex during the show, just like Onashia.

Throughout Simoun, Dominura gradually became ill, both mentally and physically. When she went back in time with Limone, she became bed-ridden, her symptoms worsening. However, she decided to still fly to different timelines with Limone. At the end of the series they walk off, but not before it is seen that Dominura's skin is shedding gold, much like Onashia's skin did in the future. She is progressively becoming ill due to her decision to not choose a gender. Since she is chronologically around sixty years old, this probably happens to her during the show and not to the other gender less characters due to the length of time in which Dominura has refused to pick a sex.

At some point in time, Limone likely died, leaving Dominrua all alone, but still very old, due to her body's way of aging. She would've been involved with the early founding days of the Simulacrum empire. However, the fact she was gender less would have soon become prevalent due to her age and abilities as a Sibylla. Thus, she could have been forced to become the Guardian of the Spring as "atonement", as it is mentioned that the religious faction of Simulacrum was strongest in the beginning of the empire.

When Yun saves Onashia, she later cries out "Dominura", as if realizing Onashia's true identity.