Aster is a character that mentioning in Omega Ruby/Alpha sapphire. She is never seen in any part of the game even though!
Read how she is mentioned in game,
Aster is first mentioned at the beginning of the Delta Episode by Zinnia, who swears to Aster that she will protect the world from an incoming meteoroid.
Near the end of the Delta Episode, Zinnia reveals to the player that she and Aster used to be together all the time before the latter was lost.
According to an interview with Shigeru Ohmori in the June 2015 issue of Nintendo Dream, Aster was a person with special powers who held the position of Lorekeeper before Zinnia, much like Aster in Pokémon Adventures.
Complete details about this character is still unknown, as she never took her appearance in any games/anime. The only known Pokémon she owns is Noivern.