Eyes? Lungs? Pancreas?
Demonic Abilities were weird by Anfang's standards.
Most of them were so volatile that they defied the norms. Some Sin Series Abilities could even upset the balance of power in favor of their users.
This was usually the case because the demonic Abilities took users' emotions, wills, and intents into account. Since the emotions were unpredictable, the same unpredictability would manifest in the respective demonic Abilities as well.
Reva Rain's Ability allowed her to cast a wide debuff AoE on her opponents drenched in Reverse Rain. It also allowed her to heal from severe injuries. The rate of the healing was dependent upon the emotions of pain and suffering the opponents felt while being affected by the AoE Ability.
Reva could also heal her allies using Reverse Rain. It was she would have to cause equal amounts of suffering, if it could be quantified, that is, to make up for healing multiple rankers from her team.