Starshine Twinkle is a student of Empress Lunaria, And magical prodigy. However, unlike Amaryllis.
She's quite shown boastful and condescending toward others ponies, Despite Empress Lunaria's lessons about "humility"

Empress Lunaria shows Starshine the Crystal Mirror

In it, Starshine sees herself as a powerful Unicorn one who could rule Equestria. She's briefly sees something in the mirror, but Empress Lunaria guides her away from it before she gets too curious

During the weeks that follow, the crystal mirror is all Starshine Twinkle can think about, But Empress Lunaria repeatedly tells her that she'll learn about the mirror and other lessons "when She's ready"

Starshine Twinkle berates her teacher for hiding such magic from her, And demands to be made a "Empress"

Empress Lunaria refuses. Saying that being a Empress must be succession, and removes Starshine Twinkle from her position as her student, And banishes her from Unicornia palaces

She has the Unicorn guards escort Starshine out of the palaces, But Sunset overpowers the guards and escapes through the mirror. As Empress Lunaria has the mirror moved to her throne room

Starshine Twinkle emerges in the humanoid World as a Teenager girl…