As in all Pokemon Games, The Alola Region too has a number of recurring Supporting Characters you will meet up with throughout your journey. Usually these are either friends or rivals of your Main Character.
You will first meet Hau after receiving your Starter Pokemon, a boy who will later become your friend and official Rival. He is very young and carefree with a large appetite, but he too is ambitious enough to challenge and eventually defeat the Island Challenge.
Kukui is the Professor of The Alola Region. Unlike a number of previous Professors, Kukui is much more interactive throughout the entire main storyline. He even loves to Battle, being the Masked Hero of Royal Avenue and the last person you must face in order to become Champion after defeating the Elite Four.
When you first meet Lillie, so much mystery surrounds both her and Cosmog. However, you will eventually find that she is the sister of Gladion and daughter of Aether Foundation Head Lusamine. She saved Cosmog from Aether Paradise and her mother, finding her way to the shelter of Professor Kukui and his wife. She will travel virtually alongside you throughout your entire journey.
Gladion is the brother of Lillie, yet he's not your friend nor a rival. Like Lillie, he ran away from Aether Paradise after saving a Pokemon the foundation had made and were trying to use similarly to Cosmog, Type: Null. Unlike Lillie, however, who found her way to the safety of Kukui, Gladion ended up getting mixed up in joining Team Skull.