She went to Mahalo Trail on Melemele Island, the place where you first met her. She will be there together with Nebby, wich has evolved into Solgaleo in Ultra Sun and Lunala in Ultra Moon. This will be your chance to encounter it (not shiny hunt it because of the lock) and - after you caught it - fuse it with Necrozma.
I don't know where she goes after you catch Nebby, but after episode RR, she will be at Mahalo Trail on Melemele. After you catched Nebby, you can return there to also capture Tapu Koko.
Hope this helps!