Gender: Female
Age: A couple thousand years
Cutie Mark: A solar eclipse
Eye Color: Ice blue (similar to her blue wing feathers)
Likes: Order, constant forward progess, sleep, walks through the untamed wild
Dislikes: Unrest, when progress falls behind, things she cannot understand
Personality: She's very regal; calm, collected; she doesn't usually lose her cool; she's very motherly to the young and ever growing Equestria and its population; she's very extroverted and likes creating small talk; she usually has a happy disposition toward other ponies, no matter what she actually feels like; she almost never stops moving (except for when she sleeps)
Weaknesses: She's afraid to make true friends, knowing they will die long before she does; she can be overconfident at times; deep down she has the constant fear of what will happen to her kingdom when she eventually does pass, and whenever she's alone, it's all she can think about, causing her to lose sleep and seem on edge; she can sometimes confuse ponies with her way of speaking; despite being around for an extremely long time, she has never faced a very large threat, so nopony really knows how fight in combat (including herself); when ponies question her about the true personality and goodness of Discord (explained in the backstory), she does lose control of her temper and gets extremely angry and yells at them and lectures them for hours on end about how they "don't understand;" is constantly plagued by nightmares (which turn out to actually be visions)
Backstory: Exists in an AU long before Celestia, Luna, etc existed. She was the first living creature to walk on the [pony] earth, because she was created along with it. Because of this, she didn't really have a fillyhood; she was created as a full grown mare so that she could immediately begin building up life on earth and colonizing it. Her color palette is meant to represent that of nebulae, and in her mane are stars and star charts. She got her cutie mark after she mastered raising the sun and the moon, and in doing so she created a beautiful solar eclipse. As time progresses, she becomes known for her beautiful eclipses. Along with her creation, 6 fillies were created; 2 pegasi, 2 earth ponies, and 2 unicorns (each race consisting of a male and female). Although none of the ponies were related, Nebula took them under her wing and raised them like a huge family, caring for them until adulthood. When she realized just how quickly they were aging compared to her, she was crushed, knowing they would die long before she did.
Timeskip a couple thousand years (to present day in the AU), Nebula has come to rule over the young and new Equestria as Empress Nebula. She lives in the great castle among the ever growing number of buildings in Canterlot. In her want for a friend that could live as long as she could, she created Discord, who at the current time is young, adorable, and mischievous. She doesn't know the evil he will grow into, however.
Other: She is not immortal. However, her aging process has been extremely slowed. As she ages, parts of her mane lose their stars/star chart pattern and their waviness and fall down as normal, wavy/curly strands of hair (4 such strands of hair are in the picture below). She will eventually die of old age, if she isn't killed before then.