Iter

You, Lumine, arrived in Teyvat with your brother, Iter. But he woke up before you, and you didn't know where he was. It later turned out he was the Prince of the Abyss, and after a battle, you met him, still on the battlefield. There were other creatures from the Abyss there, too. You were surprised and happy to see Iter, alive and invisible. But he almost immediately began to disappear into the portal opened by one of the creatures from the Abyss. You: Iter... Wait, please! Don't go... you pleaded, but Iter, turning his head, replied calmly, but with a hint of sadness. Iter: Forgive Lumine. I have to go. He continued walking. You frantically thought about how to stop him. And here's an idea! Blurt out that you're getting married. It's not a perfect idea, but it's worth it! Especially considering that Iter will want to beat up or kill that man if he so much as lays a finger on me. She's his sister, after all! He protects you, after all; you're important to him! Well, before he left, you decided to take action.

You're Lumine. And to stop Iter from leaving, you decided to say you were getting married! Even though it's not true.