Solgaleo

Walkthrough: After saving both Guzma and Lusamine from Ultra Space, Kahuna assists in getting the ill Lusamine back to Aether with the help of Guzma. Before Lillie can join them, Solgaleo (Nebby) makes a motion that Lillie believes is a direct message towards her, wanting to still travel alongside her. However, Lillie doesn't think it's the best idea for her, someone who isn't even a beginner trainer, to be traveling with something as strong as a Legendary Pokemon. Instead, Lillie asks if you would capture Solgaleo and let it travel with you for the rest of your journey, being an experienced trainer and all. You are forced to capture Solgaleo, and your adventures on Poni Island will officially end after adding it to your party.

Solgaleo (Level 55) - Sunsteel Strike, Cosmic Power, Crunch, Zen Headbutt

After capturing Solgaleo, which you have no choice otherwise but to do, you and Lillie will go your separate ways for now. She will end up returning to Aether to look after her weak mother, while you head back to Ula'ula Island and Mount Lanakila to take on the newly created Elite Four.