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.