Hau’s favorite food in Pokémon Sun and Moon is malasada, a famous fried treat of the Alola region.
Hau is the cheerful grandson of Kahuna Hala and serves as the player character’s friendly rival. His enthusiasm for malasadas is a recurring character trait: he frequently talks about visiting malasada shops and is often shown eating them. The game even uses malasadas as items that can affect Pokémon affection or cure certain status conditions.
In the Pokémon world, Alola’s malasadas are inspired by the real Portuguese pastry of the same name. Malasadas are made from enriched yeast dough, fried, and commonly coated with sugar or filled with flavored cream. They became especially associated with Hawaii after Portuguese immigrants brought the pastry there in the late 19th century.
Hau’s love of malasadas can be confused with a general Alolan food reference, but the question asks specifically for the character whose favorite food is malasada. That character is Hau.