The Mark of the Covenant is an emblem that is placed on the chest of those who have signed a pact with Hell Girl: a reminder of their agreement.
When a person contacts Ai Enma to seek vengeance on others, Ai delivers a straw doll to the person and explains to them the price for their services: that person's soul to be delivered to Hell when they die at the end of their natural life. After the red thread on Wanyuudou is pulled and the person ferried to Hell, the Mark of the Covenant appears on their chest as it serves as a constant reminder to the requester of what they have done and where their fate lies. Some initially do not take the symbol seriously but others know it as their doom. One example is of Fukumoto, a man who sent his best friend to Hell for raping his wife and her subsequent death. He confesses to Hajime Shibata that no matter what he did in life, work, gambling, even religion, nothing can cleanse your mind of the Mark and what it means.