Bonsley is the child of blueberry ( underswap sans ) and underswap papyrus. Though, it was not meant to happen. It was made by soul mixture, NOT blueberry X underswap papyrus.
He is mostly looks like underswap papyrus except as a sans. He has a blue hoodie with a yellow star clip on the hood. He has smoke on his cheeks as well as his yellow shirt. His skull colour is white with no marks other than smoke. He has no shoes or anything on his feet. He has blue sports shorts that has some smoke staining on it.
Papyrus and Sans' souls were mixed and created another being. After he got mature, he left his family and ran to live somewhere else, soon being found by error and taken away from that AU and moved to his place. He was never killed, but just kept there to get blueberry and UnderSwap Papyrus to come.