The Fat Friar was a wizard, who attended Hogwarts School of Witchcraft and Wizardry in his youth, and was Sorted into Hufflepuff. He was a jolly man and very forgiving. He devoted his life to religion and, after his death, returned to the school as the resident ghost of Hufflepuff House.
The Fat Friar is the ghost of Hufflepuff house. He is a portly, jolly spirit in clergyman's garb. Not much is known about him.