Notes:

Context: after beating Geeta, she'll greet you as the new champion. However, she'll let you know there's one more person who wants to fight you. I did not exactly get if Nemona had already beaten the league herself or what because the stream was lagging, but it seems she did beat the league ahead of you. The difference with Lance from Kanto is that Geeta talks to both the MC and Nemona as if they were champions separately, there's no "you must fight each other to decide who's the champion" situation.

Before getting into battle, Nemona will invite you to be her rival. You can decline. The more you decline her offer, the more depressed she gets, she does not have a stock reply that repeats over and over until you say yes (pic related is a capture of her reaction to the third "no"). Reetardkun finally said "yes" out of pity, it was getting really awkward.