For a story about mysterious occurrences and a resurrected dead person, a ghost is natural to appear. Technically they are not called 'ghost' in my series, but in essence they are the same.
I like ghost stories. I like the idea of ghosts in many cultures or media that they are spirits of dead people who is unable to find peace as they die a horrible death and/or a tormented life before. Sometimes I also feel that they are soul of vindictive people unable to forgive and let go. It is all depends on the lore or story. They can make you feel sympathy or straight up terror.
I think it is more tied to human's belief in afterlife and divine retribution, which are maybe rooted in their disappointment of the justice system we have, and their own mortality. Sometimes I see cruel and/or unjust deaths real life and think what if the victim became a ghost that will assure the perpetrators never get even one second of peace? If dead people can avenge themselves, would murder rate and number of cruelty go down?
That sentiment aside, they are cool and defy logic. You can also give them cool powers and edgy appearance.
Another ghost is also scheduled for debuting in the upcoming chapter.