Rosier is the surname of a pure-blood wizarding family, and one of the Sacred Twenty-Eight. They are related to the Black, Lestrange and Malfoy families. Given these relations, as well as the participation of two Rosiers in the First Wizarding War as Death Eaters, it is likely that the Rosiers place great importance on their pure-blood heritage, disdain Muggles and Muggle-borns, and practice the Dark Arts.