En's name along with Core's forms the word "encore".
En was found on the street by Yoko Sakaki. When he was found, he was small enough to fit in the palm of Yuya's hand.
Pre Arc League Championship
After waking up from a dream, Yuya exited his bedroom to be greeted by En and Core. Yuya picked En up and reminisced about when En was a puppy, also noting that he had gotten larger and much heavier since he was a puppy.