Madoka Maruyama (丸山円, Maruyama Madoka) is No.4 of the Muto-assassination team. Maruyama is a bishōnen that is usually under the disguise of a fat, chubby ball, until Tokiko cut open the disguise with her Valkyrie Skirt. He was able to shrink Tokiko to 34 cm in their first battle, then seemingly shrink her to nothing in their second battle, only to find Tokiko still has 4 cm remaining and went into his stomach. He was defeated as his guts were ripped out by Valkyrie Skirt. His special ability is that he can draw a circle without using a compass. In the anime, he, Inukai, Chitose, Busujima and Shusui were sent to fight Victor, but lost against Victor's Buso Renkin, Fatal Attraction, and was forced to retreat. He doesn't use his fat man disguise in the anime either.
His Buso Renkin is Kakugane XXXII, Bubble Cage, a large number of wind-navigated fire balloons that will shrink the person it hits by 15 cm per hit; a person will vanish if he/she gets hit by too many bubble cages. By detonating one Bubble Cage, he can create wind force that'll drive other Bubble Cages towards the enemy. Although these balloons make laughing sounds when moving, they have no intelligence. In the anime, Bubble Cage does not shrink the target, but when it explodes, it splits into more Bubble Cages. Voiced by: Junko Minagawa (Japanese), Sam Regal (English)