If you don't wait for Shadow, Relm is in the cave on the Veldt and if you do wait for Shadow, then he is in the cave. Either way, after you complete the cave then Relm is in Owzer's house regardless of if you waited for Shadow or not. I'm not 100% sure if you have to complete the cave on the Veldt to do Owzer's house, but Relm is always in it.