Justice Devil

The Justice Devil (正義の悪魔) is a Devil that embodies the fear of justice.

Appearance
The Justice Devil’s true appearance is unknown but the Class President’s transformation takes a form of a disfigured blob of flesh with human lower torso and legs, that possesses large muscular tentacles with claws-like fingers, which are sharp enough to cut through human flesh and bones. It also has six heads, two of which are merged, with different expression of their host.

History
At some point, the class president made a contract with Justice Devil for an unknown price, because she was jealous of her classmate, Asa Mitaka for capturing Mr. Tanaka's attention due to her aloofness, and the lack of socialization in their class. Mr. Tanaka was also having an affair with the class president, but it's unknown when it started.

War Devil arc
After Asa accidentally killed Bucky, the class president and Mr. Tanaka decided to walk with her to his grave at night in hopes the spirit of him would've forgive her for killing him.

On the way there, Mr. Tanaka, and the class president cross the street, and Asa stays behind. The class president realizes it's because Asa won't cross the street without the green traffic signal and begins to berate her for it for being a goody-two-shoes. She suddenly mutates into a being made of muscles and reveals that she made a contract with the Justice Devil, claiming that she and Mr. Tanaka had been having relations, but he had a thing for her, which held them back. Because of this, the only way for them to be happy is to kill her. She also reveals that her plan was for Mitaka to kill Bucky in order to humiliate her.

The class president is later killed alongside with Mr. Tanaka by the War Devil after it took Asa's body.

Abilities
As a Devil, the Justice Devil possesses all of the standard devil abilities. These include the ability to make contracts with humans, such as that made with the class president for an unknown price.

Physical Prowess
Enhanced Speed: The Justice Devil is able to move very quickly, being able to end Asa Mitaka's life in just seconds.

Enhanced Strength: The Justice Devil was capable to slice though her classmate's skull and flesh, killing her instantly.

Devil Powers
Possession or Mutation: The Justice Devil has the ability to either possess or mutate humans who enter into a contract with them, even if they are still alive. When the contractor chooses to use their powers, the Justice Devil either manifests through their body or their body gains some monstrous mutation as an effect of the usage, changing its appearance but keeping the contractor's consciousness intact.

Trivia

 * The Justice Devil could be based more on one one’s own personal sense or “idea” of justice rather than actual justice itself.
 * The transformation of the class president shows numerous heads, as if to symbolize that the idea of justice has "many faces", varying according to each person's personal perspective.
 * Based on this, the class president found right and "justifiable" to kill someone to have her happy ending with her teacher.