Sailor Theia

Sailor Theia was the senshi identity of Serenity of OKeanos, better known as Queen Serenity. She is the main and titular character of the Fanfic "Serenity's Story" by Kyralih.

Personal History
Serenity was born on the Planet OKeanos to unnamed parents, who died when she was very young. Like Galaxia, she wandered around her planet as an orphan until she discovered she was a senshi at age ten, whereupon she was adopted by Queen Amphitrite and grew up in the palace alongside Princess Eos/Sailor OKeanos. During a conversation with Luna and Artemis, she states that her first major battle as a Senshi occurred when she was 12.

At some unspecified point after this, she met Hyperion (a knight of Amphitrite's Court) and fell in love with him.

Destruction of OKeanos.
When Serenity was 18 years old, an unspecified cult summoned Necroma (an embodiment of Chaos) into existence. Although every member of the cult was immediately murdered by Necroma, the destruction of the planet did not stop there.

Trivia

 * In greek Mythology, Theia and Hyperion were Titans as well as the parents of Helios (The Sun), Eos (the Dawn) and Serenity (The Moon).
 * She is stated as being 18 years old, in line with Naoko Takeuchi's notes that she appears to be around that age.