One day, one of the magic mirrors in Twilight's castle unexpectedly activated. Light flashed and the mirror shattered, but two tiny foals sat where none were before. They were alicorns, and even though they were very young they had cutie marks branded on their flanks. While they seemed to recognize Princess Trixie, when they saw Princess Twilight they immediately exclaimed that she was their mother. Upon further questioning, it was discovered that the two are the "twin" children of Twilight Sparkle and Sunset Shimmer from an alternate universe of strange magic, and they had been sent to this world, earning their cutie marks and their alicornhood in the process, for it was through their combined effort that they were able to reactivate and navigate the portal between worlds.
Glitter was born a unicorn, and was born to Twilight Sparkle. Quiet and introspective, she is nonetheless very curious and adventurous, and goes off on her own often while just as often getting hopelessly lost. She is very attached to her alternate universe mother, Twilight, though she hasn't quite warmed up to the idea of Trixie as a parent, and she is also fascinated with Prince Pantheo, possibly due to his own transdimensional powers. She was the one who flung her brother and herself into the void, earning her cutie mark.