It is the transformation item of Flare, an Ultra Warrior that appeared in Ultraman Story 0 manga.
History[]
The Flare Coin was forged in the Land of Light using Plasma Ore, special material soaked in the light of the Plasma Sun. It was Flare's until his death near the end of the Manga
Transformation[]
Take it of its necklace, and flips it, catching it and placing it on the back of his hand, before transforming.