From the Super Mario Wiki
Chap is an inhabitant of Flipside who appears in Super Paper Mario. The word "chap" is a slang term for a male person, and is used multiple times in Super Paper Mario: there is another, green-coloured Chap, and Flamm's nickname is "the Map Chap."
Chap thinks very highly of milk and can be found constantly slurping up the substance at his table in The Underwhere. According to Tippi and Tiptron, Chap arrives at The Underwhere each morning at noon and stays there for the remainder of the day, drinking milk in an attempt to quench his apparently perpetual thirst. (This is not literal; players can encounter Chap earlier than noon.) He apparently enjoys joking that milk flows through his veins, a series of jokes he once secretly took seriously and approached a doctor about. Throughout the course of Super Paper Mario, all Chap does is talk about his love for milk, stating that it should only be served straight up. Chap's Flopside counterpart is Chip, who coincides Chap's milk obsession with his own coffee-drinking habits (and even mirrors Chap's quirk of arriving at a store serving his favorite drink each noon and staying there for the rest of the day).