Name: Eternity / Rouge
Age: She's about 16 by appearance, otherwise she doesn't know.
Personality: She's what you call two-faced, meaning she has two personalities. Her first personal (Eternity) is cheerful, optimistic, slightly annoying, has a way of always getting in trouble, and doesn't feel remorse. Her other personal (Rouge) is sadistic, scary, dangerous, a tough cookie, manipulating, but cries in silence. If you ask me which personality is scarier, I'd say the first one.
Eternity is always wearing skirts, dresses, but never trousers. Everything she wears expresses 'happy'.
Rouge tectonically always wears darker tones and trousers. (Unlike the picture) Rouge never wears skirts or dresses. As you noticed, her hair tuns darker into a shade of red and her eyes burn with scarlet (because she cries a lot).
Bio: Rouge was the original personality, but because of her cruel past her mind self-consciously created another one. Her earliest memory was of being dragged and thrown down a cliff. When she woke up, she was saved and picked up by a kind family. However, while both of her personalities remained, her memory was erased, otherwise known as 'amnesia'. She didn't care, so continued her peaceful life in the village with the family until one day, a group of dangerous people attacked the village, putting it into flames. She fled to "Empress" Niara Eternity (as Eternity) and stayed in her favor. However, Rouge had a secret liking to the other side, to "Empress" Elizabeth.
Side: For now, "Empress" Niara Eternity.
Eternity's preferred weapon: http://fc08.deviantart.net/fs39/f/2008/357/6/e/Ruby_Bow_Fire_Opal_Arrow_by_Sunrise_oasis.png
Rouge's preferred weapon: http://media.photobucket.com/image/cool%20mythical%20swords/Yoro-san/story%2520chars/sword-4.jpg
She has both, but each personality tends to chose one of the both (as shown above).
Ability: Her strength increases like the sun. For example, at sunset her power begins to decrease until nighttime, which she is just as strong as a she was originally (without the strength increase). In the crack of dawn, when the sun starts to rise, is when she is 'almost' unbeatable.
Other: In both personals, she/both is an expert with whatever weapon they prefer originally, so she can still beat you (even in nighttime) depending on how strong you are.