HAPPY 15(フィフティーン) is the seventh indies single by THE Possible. It was released on November 7, 2007. "HAPPY 15" was used as theme song for the release campaign of the manga THE Possible - Robin to Mahou no Apron. This was their first single released after they graduated Hello Pro Egg and joined NICE GIRL Project!. The Single V DVD was released on December 5, 2007.