From the Super Mario Wiki
Walter is a person living in Flipside in the game Super Paper Mario. Married to Gladys, he is around eighty years old and his memory and strength are not as good as they used to be, according to Tippi. Walter just loves to eat food. Walter appears in the second floor of Flipside. Walter seems to like the games at the Flipside Arcade. He has high scores of 200000 in Forget Me Not and 300000 in Tilt Island. His Flopside counterpart is named Harold.