This confirms we have a second rival and that it isn't Lillie. From what we've seen, she isn't into battles, yet. If we have Rowlet, Hau would have Popplio. That means a second rival has Litten and that we battled them, otherwise there is no way we could have seen Litten yet. Moreover, based on the exp. bar shown in the treehouse demo, we should've battled at least two Pokemon because the EXP bar is too full for just a match-up vs. Popplio. However, we know this is before the festival because Pichu and many of the wild Pokemon aren't on the dex yet.