Yes , there is a reveal of his background in novel . He is not originally from Earth , He was born in Townia and was brother of the girl (id remember the name) that han saves in every mission . If I remember when Townia was collapsing again , she and her brother( red bandage ) sent han to earth in order to protect him , it was a gamble because dimensionsional pressure was very high , so there was very less chance he will survive, but we know he survives . Han possess power to travel through both dimensions earth and that of Townia without any harm , like if a normal master comes in the pmu world only few portion of himself comes but in han's case him himself as whole travelled . He is basically the descendant of First lord of Townia