Walkthrough: After helping Lillie retrieve Cosmog and returning to Route 3 with her, Hau will suddenly show up and will challenge you to a Battle before your Grand Trial Challenge.
Rowlet Starter - Level 13 Pikachu, Level 14 Popplio
Litten Starter - Level 13 Pikachu, Level 14 Rowlet
Popplio Starter - Level 13 Pikachu, Level 14 Litten
After defeating Hau, both him and Lillie will go ahead of you to Iki Town to await for you and your Grand Trial.