There is much more History between Lumine and Wanderer. As Scaramouche, he tells Lumine before he enters Irminsol in the belief he will save his friends in Tartarsuma, that they have met each other before. Actually, Scaramouche is the lynchpin of the story as he is the ONLY character who can tell Lumine ANYTHING about her brother. This happens in the middle of the game. He is also the 6th strongest (middle) Harbinger as well. The information he tells her is her brother lived with Pierro when Pierro was a royal Madge in the court of King Irmin in Khaenriah,. This was prior to Pierro becoming head of the Fatui. The use of time in the game is so vague. Both Lumine and Aether were on Teyvat perhaps several hundred years before the downfall of Khaenriah and the Catacylsm. We don't know exact ages or timelines or anything. Lumine knew Kabukimono (Scaramouche), when he was in Tartarsuma. Thanks to Pierro;s magics and spells and Dainslief's control through Irminso (Dainslief is the BoughKeeper) she just doesn't remember. Again the timeline is so fluid. Nothing is exact. Both Lumine and Wanderer, have know one another for a long, long time and now (check out Simulanka), they are coming much much closer!