Restless Ghost is based off of a Yurei, the Japanese counterparts of ghosts. According to traditional Japanese beliefs, all humans have a spirit or soul called a 霊魂 (reikon). When a person dies, the reikon leaves the body and enters a form of purgatory, where it waits for the proper funeral and post-funeral rites to be performed, so that it may join its ancestors. If this is done correctly, the reikon is believed to be a protector of the living family and to return yearly in August during the Obon Festival to receive thanks.
However, if the person dies in a sudden or violent manner such as murder or suicide, if the proper rites have not been performed, or if they are influenced by powerful emotions such as a desire for revenge, love, jealousy, hatred or sorrow, the reikon is thought to transform into a yūrei, which can then bridge the gap back to the physical world. The emotion or thought need not be particularly strong or driving, and even innocuous thoughts can cause a death to become disturbed. Once a thought enters the mind of a dying person, their Yūrei will come back to complete the action last thought of before returning to the cycle of reincarnation.
Her background is based off of the stereotypical Japanese Delinquent, wearing the Tokko Uniform and holding an iron pipe.