It's Pokémon Champion Cynthia cosplaying Rosa, your rival.

She figured you'd stop trying to run away from battling her if she disguised herself as Rosa.

Don't point out how Cynthia's 'obvious physical differences' between her and Rosa meant you weren't fooled for a second by her disguise. 

She'll get depressed because she thought this trick was actually going to work on you.

You can make it up to her after the battle by going to get her favorite treat, ice cream.