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Ghost is a demon, or a soul guider. They are harmless and kind, and loves helping people and trying new things like new foods and hobbies. 

Demons are seperated to different ranks and jobs.

Low ranking Demons (Level 1-3) are harmless and don't cause any threat. They can only interact with people when summoned.

Medium Ranking Demons (Level 4-7) are harmless but are won't hesitate to attack people who provoke them first. They mostly appear invisible to the human eye or look like humans, unless they want to be seen, humans with special abilities (like Faith, Z, and Keith) can see these entities. Half humans can see them too.

High Ranking Demons (7-10) are hostile and won't hesitate to kill/mortally wound people who they think is a nuisance. Like Medium Ranking Demons, they would appear invisible. Most High Ranking Demons are rare but most are either Demon Council members, Evil overlords, Rulers, Generals or high ranking officers.

Ranks would be revealed to baby demonlings who could read and write.