As per conventional Japanese convictions, all people have a soul called a 霊魂 (reikon). At the point when an individual dies, the reikon leaves the body and enters a type of limbo, where it sits in wait for the legitimate burial service and post-burial service rituals to be performed so it might join its ancestors. In the event that this is done accurately, the reikon is accepted to be a defender of the living family and to return yearly in August during the Obon Celebration to get thanks for its protection.
On the off chance that the individual dies in an unexpected or vicious way like homicide or self-destruction, and the legitimate rituals have not been performed, or the deceased harbored strong feelings such as craving for retribution, love, desire, scorn or distress at the time of their death then their reikon will haunt its family until they have received the burial rites or it has resolved the emotional turmoil that keeps it bound to the physical world.
It is said that the reikon has the ability to change into a yūrei which can overcome any barriers back to the physical world through a cycle of reincarnation. The yūrei then exists on Earth until it released, either by playing out the missing ceremonies, or settling the profound clash that actually attaches it to the physical plane. In the event that the customs are not finished or the contention left unsettled, the yūrei will endure in its haunting.
Source 1: Wikipedia, retrieved 09/11/2022; shared under Creative Commons and Attribution-NonCommercial-ShareAlike 4.0 International license.
Source 2: Yokai.com , retrieved 09/11/2022
Yurei