Eclipse Starlighter is the descendent of the ponies that fled from Equestria in the days of the Fall, settling in space habitats and colonies on other planets. For two centuries they have severed all connection with the surface. However, a rising movement amongst the Spacers - the Reclaimers - struggled to convince the United Council to launch a program to reclaim the surface, and help those ponies that were abandoned there. The Council finally buckled and authorised an official expedition to the surface, to gather data and materials that were to demonstrate that the surface was not quite as hostile as it was thought, and that there were indeed ponies in need of help, not just hordes of ravenous mutants. Eclipse, a major member of the Reclaimers, was appointed as the leader of the expedition. She was sure that finally she will be able to make a difference. She hoped that she will be able to help all those ponies that were left to the mercy of the TITAN.
She was the only survivor of the expedition, getting stranded on the surface when their camp was attacked by a band of well-armed raiders. By chance Eclipse was rescued by Blaze and Co. Since then she had little choice than to tag along with Blaze, looking for a way home.
Eclipse is somewhat naive and very idealistic; she is convinced that there is good in everyone, you just need to look hard enough, give them a chance. Of course, Wasteland does not give second chances, and it is often only due to Blaze's interference that Eclipse still has her head on her shoulders. She is very friendly, too, and she does not understand why Wastelanders are none too eager to accept her friendship.
In many ways Eclipse is the exact opposite of Blaze. Early on they did not get along, because Blaze's method of solving most problems boiled down to application of copious amounts of violence, but eventually she warmed up to the angry Wasteland unicorn, realizing that deep down she, too, wanted to make a difference, to make the world a better and safer place. She hopes that her influence would eventually make Blaze discover a better way to do better.

MLP:FiM © Hasbro
Eclipse Phase © Posthuman Studios
Eclipse Starlighter © me