6x19 - Codename: Kids Next Door

Heinrich uses a ritual involving sacred golden caramels to take away the qualities the Sector V operatives value the most (Numbuh 1's leadership, Numbuh 2's intellect, Numbuh 3's giddiness, Numbuh 4's toughness, and Numbuh 5's coolness). Numbuh 5 eventually teaches Heinrich that he needs to share his candy to revert to his original form; finally learning to do, it is revealed that Heinrich is actually a girl named Henrietta, whose most precious quality, her femininity, was taken away in the ritual many years ago.