They had no plans to put Cheren in the anime originally, they already had three rivals for Ash (Trip, Bianca, and Stephan), as well as rivals for Iris and Cilan (Georgia and Burgundy). Then later on, Trip got replaced with Cameron
There were a fuckton of rivals in BW, and they chose to only use Bianca from the games and make up their own characters for the rest. Then BW2 happened and they had to include the new Gym Leaders somewhere. So they made Roxie Ash's final Gym Leader instead of Drayden like they originally planned, Marlon Cameron's, and gave Cheren an unofficial exhibition battle with Ash
