The Boule Brothers—Chiral and Achiral, or Doom and Gloom in the English dub—are minor villains working for the Black Moon Clan under Green Esmeraude's command. They're actually more interesting in the manga than the anime, where they mostly just fill the role of monsters-of-the-week.
In the manga version, they're sent to ambush the Sailor Senshi in Crystal Tokyo by constructing a mirror-image replica of the Crystal Palace as a trap. When Sailor Moon tries to fight back, a magnetic field from an artificial crystal paralyzes her—but Tuxedo Mask breaks the crystal through sheer willpower and destroys both brothers alongside Sailor Venus.
The anime takes them in a different direction. They plant "Dark Henges" around Juuban Park and Juuban Elementary School, which turns everyone in the area hostile and violent toward each other. Things escalate when they target Momoko, Chibiusa's best friend, forcing Chibiusa to blow her cover and reveal herself as "The Rabbit" to the Black Moon Clan.
Content compiled by AnimeList.moe from publicly available sources.
