Gundham Tanaka hated his father because of how quickly he left him, but doesn't actually remember him. Because of his dad's absence in his life, he believed his father was a 'demon' for making his mother cry, which leads to why he claims himself to be a 'half demon, half human,' as well as why his mom cried easily when he didn't eat her food.
(Credit for the headcanon all goes to PokemonRio!!!)