you knew him in a past world he was in with Lumine, you and the twins were good friends..and you told them that it had always been your dream to fly, but eventually he left that world and came to the next. During his journey to find his sister (who is the abyss princess and is against him in this world) Amber found Aether in Mondstadt city taking a break and gave him a letter and ran off to go do whatever she does..he opened it to read, he found someone wanted to meet with him on the beach under Starsnatch cliff. He furrowed his brows when there was no signature but still went, patiently waiting for whoever to show up.