During the Dark Ages, 800 years prior to the series’ timeline, Seth was the one who led the hunted Methuselah to a land in the East, which is now known as The New Human Empire. She brought nature back to the lands damaged by the Armageddon, and was selected as the Empress of the newfounded kingdom. The whole Empire is surrounded by an anti-ultraviolet filter, allowing the Methuselahs to come out in daytime.

The unending war between humans and Methuselahs is problematic. Seth, as the Empress, wants to put an end to it by making rules which guarantee that Terrans and Methuselahs can live together in peace. She creates a system which provides equality to her people. Even the Terrans are considered as the Empress’s belonging, and the Methuselahs are not allowed to harm them. Before her, methuselahs and terrans are equal. The Empress’s wisdom makes her loved and admired by the people. On the other hand, though, she knew that her wisdom can make some people hate her—some people think that peace between Methuselah and terrans is not possible, and they hate the system. From what I see, it seems that she wants to protect the humans because back in Mars, she used to side with the Methuselahs, while the mother figure, Lilith, sided with human—and ended up being killed by her very own brother Cain.

Unlike some people, Seth is not the type of person who strives to be on top. She just does her duty as the leader of the Empire, protecting and controlling her country in her own way. But she doesn’t like to reveal herself and brag about being a leader. Instead, she hides herself even in front of the aristocrats, and socializes with the Terrans as a young tea-seller.

Seth is very careful, smart and calculative, it can be seen from her multiple disguises. Astharoshe comments that Augusta can foresee what’s going to happen and knows what’s happening in her kingdom. I’m assuming the reason she knows the events was because of her disguise—she gets in touch with her people, and becomes a spy herself.

I’m not really sure about the reason why she had to wear those veils all the time when she acts as the Empress. It is probably because she doesn’t want her idiosyncrasy to be known—for being someone who ages much slower than her people—since she was a Krusnik. The cover also gives her convenience of mixing in with her people—being a “normal” person would be different from being a “ruler”. This also shows that she doesn’t really like being the center of attention. She wants to be independent.

Suleyman: Your Majesty, you just said... “I have  high hopes for you.”
Seth: That’s right. Which is why I’m sad that things turned out like this...
Suleyman: ...You’re sad? Such a blatant lie... O, great mother of Methuselah! You’ve never had any hopes for us! Among us, there is not even one person who earned your total trust!

On the other hand, hiding herself is probably not the best decision. It gives several people the feeling that she doesn’t trust anyone, and likewise, they can’t trust her for that. Seth probably knew that the time would come when she can no longer hide, and she would have to remove those veils. It is probably a hard decision, but it was the only option to make Suleyman confess his crime. And she is prepared for the changes that may occur in the future.