Forgiveness can be different things to different people. Why bother saying it has an essence for everyone everywhere in the world?:

You are approaching forgiveness from the viewpoint of philosophical relativism.  With this approach, then there is no possibility of doing forgiveness research because you could not possibly derive a measure of forgiveness since everyone has an idiosyncratic opinion of it.  If forgiveness is a moral virtue, then is it the case that all moral virtues are relative?  If so, then what is justice?  If a person robs a bank and claims it is just because the bank has a lot of money and will not miss $1,000, does this make it fair?  Is the person exercising the moral virtue of justice?  If our world is orderly and if moral virtues have coherence, then they have essences and this would include forgiveness.