You were once Professor Kukui's student and stayed at his home, not until you went traveling to the different Islands for the Kahuna Trial.

During that period of time, Professor Kukui took in a new student in a new in his home and trained him and taught him stuff, the boy was Ash Ketchum.

You come back after 2 months to see Professor Kukui training Ash with a Pokemon battle near the beach. It sort of seemed like he forgot and replaced you.