Like Magatsuhi, they are types of energy generated by human psyches. However, the key difference between Magatsuhi and the two types of energy is the actual generation of the substances; Ideal Energy is generated by dreams and hopes, while Kegare is a highly corruptive, potentially toxic substance generated by guilt and despair. Kegare and Ideal Energy are considered to be polar opposites.
Enormous quantities of either substance are able to warp reality according to the controller's wishes; however, the process is extremely erratic and volatile, only allowing individuals of extreme power (e.g. Nyarlathotep) to unlock its fullest potential.
Persona 2: Innocent Sin
The Masked Circle's original plan revolved around collection of Ideal Energy to use it in conjunction with Sumaru City's powers to raise Xibalba and create a golden age for humanity. To this purpose, five crystal skulls were used to drain Ideal Energy from selected victims.
Joker used one to drain the energy of anyone who called but did not answer his question, another was given to a junior member of the Masked Circle in Kasugayama High School to drain the Ideal Energy of his classmates during the school festival, Prince Taurus used another to drain the assistants to the MUSES concert, and Queen Aquarius had one to drain adults. The last one was wielded by King Leo and was lost to him during the events of the Sky Museum.
Persona 2: Eternal Punishment
Nyarlathotep switched tactics in the Eternal Punishment timeline, collecting Kegare instead of Ideal Energy through the New World Order through various means. In one way, they contacted their Taiwanese mafia ally, Tian Dao Lian to abduct the Joker callers and send them to Takahisa Kandori's science laboratory to drain their Kegare by the "Joker Separation Machine" (JSM). The process has a few serious side-effects including causing the subject to lose their personality or turning them into a vicious demon. After the JSM technology was mastered, which reduced its side-effects, the New World Order convinced people through various means to assist Ginji Sasaki's seminar, which was a trap designed to siphon the Kegare of the audience. Chizuru Ishigami describes Kegare as a negative emotion such as sin or jealousy.
The drained substance was collected, and Sasaki was forced to defend it from a violent shootout. Predictably, the Kegare tanks were shattered and he was doused in the contents, transforming him into another victim of the JOKER curse. The tanks the Order had already managed to collect were pumped into Earth's fissures, where the imbalance between Ideal Energy and Kegare would force the sleeping dragons below Earth's surface awake, triggering an Apocalypse Nyarlathotep hoped would be enough to destroy all existence.
Kegare can be rendered in kanji as either "穢れ" or "汚れ", the latter can be also pronounced as "yogore" and is more common in actual usage. Both kegare and yogore mean "filth", but kegare is more about spiritual or mental impurity (see also Magatsu.) While yogore primarily associates with physical blemish.