Warning: Spoiler Alert! Do not read if you don’t want to know what happens in Episode 15. If you haven’t watched the episode, watch it now:
Rockruff runs out of the house every night to spar with a group of Pokemon, including a Magmar that keeps defeating him. Ash sees that Rockruff really wants to win and starts to train him. After a lot of hard work, Rockruff finally defeats Magmar. In the end, Ash and Rockruff grew close and Rockruff is caught by Ash, joining his group of Pokemon friends.