Her parents are no-one important. She was found on the doorstep of the Ponyville orphanage with a broken horn and a note explaining her parents didn’t want to raise a broken unicorn. They did name her, however. Briste (I guess just to rub it in her face, tho, since it is Gaelic for broken). She was beat so much as a foal that she eventually was blinded and unable to open her other eye. She also was scarred all over. So she ran off, eventually being found and taken in by Twilight and Pinkie. She was renamed Asteria Sparkle. She possesses red magic, something rumored to be a dark and cursed trait. For this reason and since she can barely do any magic, Asteria only ever uses magic for defense.
 
Among her injuries and scarring is:  
Broken horn  
Blind left eye  
Paralyzed right tear duct and eyelid  
Extremely weak hooves  
Part of her ear is missing  
Scarred face in general
 
Despite all this, she is now a very happy pony among her adopted family. Also, I’m not big on her design so this is most likely not her final design. Also, she looks too much like Skia.