Mur Mur, alternately known as Muru Muru or Murmur, is a demonic entity bound by Deus Ex Machina, god of time and space, to assist in overseeing the Survival Game. Her chaotic nature necessitates power-restraining bracelets. She manifests as a diminutive, childlike figure with dark skin, lavender hair nearly as long as her height, and purple eyes, marked by faint red circles on her cheeks and a dimensional origin symbol (either "1" or "2") on her forehead. She typically wears a colored vest (pink or light blue), white briefs, oversized leggings, blue shoes, and carries a golden trumpet. Following Deus's death, she adopts a tattered black cape and a smaller version of his crown, covering her forehead symbol with an X-shaped bandage.

Her personality mixes playful mischief with calculated manipulation. She displays childlike tendencies, reading shoujo manga, enjoying corn and rice cakes, and disguising herself for amusement. Prioritizing personal entertainment over loyalty, she views the Survival Game as a diversion, secretly manipulating events and participants—particularly favoring Yuno Gasai. She harbors contempt for humans, directing antagonism toward Aru Akise, Deus's Observer. Her true allegiance lies with the first-world Yuno Gasai, who ascended to godhood after the initial Survival Game. To serve this end, Mur Mur orchestrated a dimensional infiltration by imprisoning the second-world version of herself and assuming her place, unknown to Deus. She then manipulated key events, convincing the first Yuno to time-travel into the second world and provoking Keigo Kurusu's betrayal to accelerate the game.

Later, in the third world (*Redial* OVA), the local Mur Mur initially blocks Yuno Gasai from reaching Deus's realm but is thwarted by Minene Uryuu. The disembodied voice of the first-world Mur Mur guides the third-world Yuno, transferring the original Yuno's memories to her. This enables the original Yuno to resurrect within the third Yuno's body, facilitating her reunion with Yukiteru Amano.

Her abilities encompass teleportation across dimensions, time travel, memory manipulation, and environmental control within the Akashic Records. She can revive the dead by rewinding their personal timelines—a power exceeding Deus's capabilities. Removing her bracelets unleashes a monstrous transformation granting enhanced strength and speed, though it rapidly drains her energy. Despite her power, she strategically avoids direct confrontation with Deus.

Her name and traits evoke Murmur, a demonic duke of hell from the *Lesser Key of Solomon*, associated with philosophy, necromancy, and depicted with a trumpet and crown.