Aging Devil

The Aging Devil is shown to be apathetic to any forms of life, both its own and that of others, as described by Fumiko, but seems to detest youth, most likely because they do not fear them. However, it is shown to possess an amicable relationship with Public Safety, evident by it complimenting Yoshida for subduing Pochita. Its hate for youthfulness led it to demand the sacrifice of thousands of innocent children in exchange for being eaten by the Chainsaw Devil. It also seems to be rather lazy, expressing annoyance at the request to "hit" the Chainsaw Devil. It also tends to get annoyed easily, such as how it got frustrated with the futility of Yoru and Pochita's attacks. Despite this, the Aging Devil says it wishes for humans to live longer so that their minds can reach new culmination of thoughts discover new concepts and possiblities. The Aging Devil appears to have some knowledge regarding how Denji defeated Makima, as they tease Pochita by calling the act “romantic” when they offer themselves to be devoured in a similar way.

The Aging Devil is a tall devil with a thin and gaunt physique, resembling a withered aggregate of flesh with several holes in it. The largest hole is placed right in the center of its chest, where the heart should be. Its face, which emerges from the split head of an old person with long hair, is a smooth surface with only a mouth, connected by several strings to the two halves of the previous face. The Aging Devil's feet have raised heels, similar to stiletto shoes. It sits on a throne made of flesh and bones, with human fingers decorating the back and tongues, mouths and teeth protruding from the base which is supported by the backs of two naked men with their hands cut off.

The Aging Devil is described as a transcendent being — one of the "Primal Fear" Devils that have never experienced death due to being so powerful; embodying such a concept, it can be assumed that the Aging Devil is almost as old as humanity itself. The true extent of their power is still unknown, but they are powerful enough to overwhelm the Chainsaw Devil with a single, powerful punch and easily stop a bullet from one of Yoru's most powerful weapons (capable of even overpowering the Chainsaw Devil himself). right|thumb|Public Safety sends a message to initiate their contract with the Aging Devil. Naturally, it possesses all of the standard Devil abilities such as blood consumption, contract creation, etc. They appear to have made a contract with three Public Safety agents: by cutting off a finger each and writing the word "HIT" on the mirrors in blood, preceded by the names of body parts (cervical spine, neck and vertebrae, probably additional body parts to be sacrificed), the Aging Devil would proceed to strike its enemy with a single, powerful attack. It is possible, however, that its number of contracts is much higher, as it tends to transport people who do not wish to age into the Aging's World as part of a contract. The Aging Devil later utilizes another contract with several other Public Safety agents, one that involves using their bodies as weapons in exchange for being spared from the Chainsaw Devil's attacks. From there, the Aging Devil can sacrifice any organ from the agents to continuously cleave Pochita to shreds. Fumiko Mifune states that the Aging Devil, due to having no attachment to life, is ready to make another contract with the Japanese Public Security and Government: in exchange for killing 10,000 children with Japanese citizenships, between the ages of infancy and nine, in front of mirrors, the Aging Devil will let itself be devoured by the Chainsaw Devil without putting up any resistance, removing "Aging" as a concept from the world. It later reveals that it wants humans to live longer so they may reach the culmination of thought on questions beyond how to escape Aging's World, and discover new concepts and unseen possibilities, even if it comes at a cost such as its death and consumption. The Aging Devil made a contract with Yoshida, Denji and Asa Mitaka, in exchange to never fight each other again, and for Yoshida, Denji, Asa, Yoru and the Aging Devil's victim to leave Aging's World.