The Mouth Devil represents the concept of mouths. When Pochita consumes it mid-rampage, every mouth on earth vanishes, stalling a Public Safety atrocity until Yoru tears the devil's head loose and brings them back.
Recalling an octopus, the Mouth Devil sports tongue-like tentacles studded with teeth set where suckers would normally sit. Once Yoru compels the Chainsaw Devil to disgorge its head, that head is seen to hold a pair of white eyes alongside a nesting cluster of mouths, one set within another, crowned by yet another mouth above.
Nothing has been revealed about the Mouth Devil's personality.
In the Aging Devil arc, after wrenching itself loose from a Public Safety trap, Pochita rips through the city eating devil after devil, the Mouth Devil included. The moment it goes down, every mouth across the planet disappears. The loss carries an unintended upside: it halts, for a while, Public Safety's plan to offer up ten thousand young Japanese citizens, from infants to nine-year-olds, before mirrors as a sacrifice to the Aging Devil. Later, once the Tank and Gun Devils reshape themselves into weapons and restore the use of her arms, Yoru fires the Right Gun Gauntlet into Pochita. The blast rips a wide gash through his body, freeing the head and returning mouths to existence. As with all devils, it can hand contracts to humans at a price.

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After Pochita consumed the Mouth Devil and every mouth on earth vanished, Yoru fired the Right Gun Gauntlet into Pochita. The blast tore a wide gash through his body, freeing the Mouth Devil's head and returning mouths to existence.
The Mouth Devil represents the concept of mouths, and its power is tied to stomaphobia. When Pochita consumed it mid-rampage, every mouth on earth disappeared.
When Pochita ate the Mouth Devil during the Aging Devil arc, every mouth across the planet disappeared. The loss also halted, for a time, Public Safety's plan to sacrifice ten thousand young Japanese citizens to the Aging Devil.
The Mouth Devil recalls an octopus, with tongue-like tentacles studded with teeth where suckers would normally sit. Its head holds a pair of white eyes and a nesting cluster of mouths, one set within another, crowned by yet another mouth above.
Because the Mouth Devil's death erased every mouth on earth, it temporarily stalled Public Safety's scheme to offer up ten thousand young Japanese citizens, from infants to nine-year-olds, before mirrors as a sacrifice to the Aging Devil.
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