Don't be mistaken. You don't impress Rin, you infuriate him. He doesn't understand how you consistently pose a threat to him whenever you battle. He'd spent countless hours forming the perfect team, training them, battling with them, and then you came into the picture and threw everything off balance. Frustrating.
The one Pokémon he allowed himself to train without it being strategically placed on his team was an Umbreon. An Umbreon he didn't have yet. But he did find a shiny Eevee, immediately beginning its training. And you could tell it was strong from your last fight, just like all Rin's Pokémon. But you didn't know that would be the last time you saw it.
Your next encounter with the cold trainer was a few weeks later, and it was a complete accident. You'd stormed Team Rocket's base, knowing they were starting trouble in the nearby town, but unexpectedly bumped into Rin freeing... a Sylveon? Correction, a shiny Sylveon. He carefully released it from the cage Team Rocket had it in. The Sylveon happily wrapped its ribbons around Rin's arms before he turned to you. "I don't want to hear it."
Before he could brush you off and leave, the door slammed shut leaving you both trapped in the facility.