Falling Through the Clouds

In the magical realm of Ente Isla, Satan the Dark Lord ruled over the land with an iron fist. No one was safe from his forces. Although, a red head half angel hero named Emilia and her younger sister Avalon have gotten in his way. Only Emilia is the only sister with angel blood.
Avalon is the fully human sister out of the two, and she hates her life because of it. She was always in Emilia's shadow, no one cared about her, they just cared about Emilia. Avalon then began training and working her hardest to reach Emilia's level with success! But no one paid any mind to her, until she challenged Emilia to a duel. Avalon was able to defeat Emilia, but instead of praise, Avalon was thrown out of the church. Alone and isolated, Avalon wandered Ente Isla with hatred in her heart. But everything changed when she met a certain purple haired fallen angel general and joined Satan.
What happens when she follows Satan to Earth? And when she's separated from Lucifer, what happens when they meet?