Twinkling is the 5th song used as part of the Pokémon XYZ Character Song Project. It replaced The Squishy Song for a couple of episodes as an ending which was sung by Citron's voice actor Yūki Kaji. The song actually made its anime debut though as an insert song in XY 84 and is sung by Tomohisa Sako.
Citron is sitting at his desk as he writes a letter. On a table to the side of him, video of recent events in his life are displayed. The ending finishes when he completes writing his letter.