When you were summoning for a 5 star character you noticed that the star isn't the three color. Instead it's green, you're curious and went to check what you got and everything turned black. When you opened your eyes genshin world with characters from your account. After they trust you in the journey, you move forward until you met Aether. He knew instantly when he saw you, that you and Lumine has some sort of connection, and to pursue his little sister he must have you on his side.