Rebel is a "Manic Entity" created by the severance of Shalom's emotions from her consciousness during the procedure that made her into the Hush of Paradeisos. After being separated from her, Rebel was trapped beyond the Observatorium rift in Deep Mania. Over the next nine years, Rebel was constantly subjected to the corruptive Mania energy beyond the rift and mutated into an enormous, monstrous consumed by rage and agony, which some referred to as a "demon". It was named "Rebel" by experimenters at the Ring Research Institute due to Shalom's aggressively rebellious nature before she was subjected to the procedure.
In addition to its role as an observation point for the Hush system, Rebel is also the source of Shalom's Sinner ability. This ability, also referred to as Rebel, allows Shalom to utilize the Mania inside other people to overwrite reality in such a way as to fulfill that person's desires. This is most often expressed as a form of healing. As Shalom explained to Rahu, as long as Rahu had the will to continue living, reality would not allow her to die. This ability is not limitless, however, and only grants Shalom control over emotions that are the same as hers: i.e., the will to live (Rahu), belief in justice (Christina), or the desire for revenge (Coquelic). When Shalom became the Hush, this ability was further enhanced with the Mark technology, allowing Shalom to use both powers simultaneously.
When Shalom drew Coquelic into the Observatorium at the end of the Blockade Operation, Coquelic reached beyond the rift and freed Rebel from its imprisonment. The rampaging Rebel destroyed the Observatorium, deeply contaminating it with Mania, and triggering a backlash from the Rules that left Shalom comatose.
The Nightmare Clan/Speculation
Rebel bears strong similarities to several other mysterious entities, which may help to shed light on the nature of some unexplained phenomena. Shalom refers to Rebel as "grotesquely oppressing its own kind" while trapped and suffering in Deep Mania. Although she does not expand on what its own kind may be, she is likely referring to the "nightmare clan".
Rebel's appearance strongly resemblances that of Hecate's Nightmare, and Hecate's history of suffering a procedure to remove her emotions also suggests a common origin, with Nightmare similarly representing a dangerous incarnation of Hecate's volatile emotions. Additionally, during the Eternal Nightmare events, the Chief interacts with a race of creatures called the nightmare clan. Although living within the Eternal Nightmare, they are harmless, even forming villages and engaging in trade. However, the event also features a boss called the Exiled Nightmare, which is described as "A member of the nightmare clan who's been exiled by its kin for its cruel nature and lack of sanity." Ordinary members of the nightmare clan are small and shadowy, but this Exiled Nightmare is a hulking, monstrous humanoid very similar to Rebel and Hecate's Nightmare.
The existence of the nightmare clan, an apparently sane and peaceful society existing within a deeply contaminated spiritual realm, raises significant questions about the nature of Deep Mania, Apostles, and all other Mania entities.