Basically, after refusing to leave for the sake of saving Nick/Captain Rainbow, Hikari sacrifices herself by knocking him out and putting him on to the Kirarin, and waves at him as he flies away, crying. She's glad for Nick's safety though and knows Nick has earned this fate, because of all the deeds he's done for the rest of the islanders, who are now gone from the island. Enter a very sweet yet melancholic piano melody, as the Shadows take over the island, sealing the shocking fate of Hikari.
Another one which eventually turns into a heartwarming moment at the end, but is still sweet for fans of those Nintendo characters appearing in this game: Nick sacrifices himself for Hikari by knocking her out, putting her into Kirarin and letting it fly away while he sternly looks at the sky. He had to make those obscure Nintendo characters realize their dreams and wishes, the reason why they came to Mimin Island in the first place. And indeed, helping them out instead of himself to the point of sacrificing himself for better futures for all the former heroes really shows the true nature of the term "hero".
This seems to be what all of Skip Ltd's games are about: helping everyone in need, despite their personality, and no matter how serious or complicated their problem is.
The reveal of Kirarin being actually dead Mimin. Hikari herself is serious and very gloomy when telling it to the remaining people in the collapsing island.