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I think Kukui was The Masked Royal's old tag team partner or something, that's why he knew how to get the mask back to him and is probably why Kukui is so fit. Also Kukui uses a Midday Lycanroc while Masked Royal uses a Midnight Lycanroc which seems like it could be a clue. I think Kukui probably got booked to lose his mask in a mask vs mask Lucha de Apuesta, putting over Masked Royal as a threat and allowing Kukui to pursue Pokémon studies. It'd also explain why Mo thought it was Kukui's mask, it was probably a similar design to the Masked Royal's.