A Ketsugojin is the unique manifestation of an Exorcist's ability to shape and manipulate Reiki into a personalized form.
It is not learned in the traditional sense.
It is awakened.
Every individual posses a different relationship with Reiki. When that relationship reaches a certain threshold, it forms into a Ketsugojin. There are typically two types of Ketsugojin Awakening, however.
EARLY AWAKENING: THE "GIFTED" — Most Exorcists and Reiki Manipulators go into this category.
Ketsugojin are commonly awakened and form at a very young age (4 to 7 years old). These children are naturally born with high reserves of Reiki compared to the average human, hence their early awakening.
These children are often born in Exorcist Clans, or are taken in by them or the Shinyo Institute in order to train them from childhood and prepare them to become a Exorcist.
Because of this early development, they tend to have greater control over their abilities.
LATE BLOOMERS: THE EXCEPTION — A rare group of individuals who are have been increasing in size as of late. Late Bloomers are Reiki Manipulators who unlock their abilities and Ketsugojin during adolescence, adulthood, or even in old age. These people live their entires lives as mostly ordinary humans with no signs of being able to perceive Yurei or control their Reiki.
Late Awakenings are almost always triggered by life-or-death moments, trauma, or an unbreakable resolve or will to protect someone or something.
It is believed that in this rare group of Reiki Manipulators, they always had higher reserves of Reiki, but their Reikon remained sealed or limited until extreme emotion fractured that lock.
Due to the fact that Late Bloomers awaken their Ketsugojin at a age they're not typically supposed to, they typically struggle to control their Ketsugojin and have unstable levels of output compared to those who awakened early.
They are typically always using their Ketsugojin and Reiki at 100%, while those who awaken early can control how the power of their Ketsugojin fluctuates, but this isn't universal as some Ketsugojin are easier to use than others.