along time ago, in the death's graveyard, life and death were having a battle to see who would be banished from existance.
eventually, the battle got so rough that the two of them simply merged together and caused a huge explosion and destroyed the lower worlds
after the explosion, it woke up, it is a combination of life and death in one person, because of the mix, he was still human but couldn't do anything by himself, so his only option was to take form with another person
he first clung to a man in a green jacket and followed him around for a few hours. but later on he realized that the man had no home. since the shadow didn't want to starve so he left the old man
next he chose a rather rich man he found on the streets. he quickly found out that he got into fights rather frequently, so he left him as well
finally he found the perfect person for him, the person was not mean and very nice and was not rich nor poor, he was just right and stuck with him ever since. the lesson that the shadow learned was pick a friend that is right for you