Regulus as he appears in a photo in Harry Potter and the Half-Blood Prince

Regulus Arcturus Black is a Pureblood Slytherin at Hogwarts school and a canon character in the Harry Potter books.  He is the younger brother of Gryffindor Sirius Black.  Unlike his brother, he is very loyal to his Pureblood and Voldemort-supporting family, seeking the help of Lucius Malfoy and Severus Snape to become a Death Eater.