Kirin's service usually comes in the form of achieving justice or righting wrongs against the weak. When she forms a contract with a mortal, she clasps their forearm and leaves behind lichtenberg scars. Her payment is somewhat cruel though: she takes the feelings and memories of the original grudge or injustice. Someone who seeks revenge against a murderer might find they don't care about the outcome by the time the deed is done. So a human will have to weigh which is more important to them- that justice or revenge is achieved at all or their reason for caring about it in the first place.