Details: After defeating Kukui and been crowned the very first Alola Region Champion, you are congratulated by him and the other members of the Elite Four. Afterwards, a celebration is held for you back in Iki Town. Everyone you have met along the way join in the Festival, including all the Trials Captains, Kahunas, Elite Four Members, and your Mother. Even Lille shows up to celebrate your accomplish.
About midway through the Festival, Lillie asks you to break away with her to the ruins just north of the town, hearing the bridge that was first destroyed when the two of you first met now having been repaired. The two of you go all the way to the shrine for Tapu Koko. Lillie gives thanks to Koko for watching you along the way and for even having met your character. She then asks you to take the Stand, and after touching the pedestal, Tapu Koko appears and you go into Battle with it.
Whether you capture Koko or not, you and Lillie will quickly return to the others before they notice the two of you having left. Shortly after arriving back to the party, Lillie tries telling you what she plans to do with both life, her journey, and her mother. But before she can spit it out, more fireworks go off that capture everyone's attention. Lillie smiles it off, even without having been able to tell you...