As in all Pokemon Games, The Alola Region also has a number of recurring Supporting Characters you will meet up with throughout your journey. Within the Alola Region, there are four characters that fall into the category of your supporting cast. For the most part, these four characters will either be your friend or foe (or both by the end of the Main Story).
Supporting Characters
You will first meet Hau after receiving your Starter Pokemon, a boy who will later become your friend and official (sort of) Rival. He is very young and carefree with a large appetite, but he's 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 one of the many challengers to later battle you for your throne of Alola Champion.
When you first meet Lillie, so much mystery surrounds both her and Cosmog (Nebby). However, you will eventually find that she is the sister of Gladion and daughter of Aether Foundation Head Lusamine and Poke Pelago Founder Mohn. She saved Cosmog from Aether Paradise, 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 older brother of Lillie, not being a friend or rival to your character. 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 joining Team Skull for shelter.