Regulus Arcturus Black
"I know I will be dead long before you read this but I want you to know that it was I who discovered your secret."
-- Regulus Black in his note to Voldemort (HBP28)
Regulus was the younger brother of Sirius Black. Unlike Sirius, Regulus was favored by their parents because he shared their overweening pride in their heritage and their belief in pure-blood supremacy; indeed, Regulus became one of Voldemort's Death Eaters around 1977 at the age of sixteen.
Sirius thought that Regulus was killed by Death Eaters because he panicked and tried to leave. In reality, Regulus began to have a change of heart when he saw first hand what the Dark Lord was doing.
In 1979, Voldemort demanded the use of a house-elf, and Regulus sent Kreacher, a family elf that he was very fond of. However, the elf was left behind to die by Voldemort on the secret island in the sea cave where he’d hidden the locket Horcrux. Regulus had commanded Kreacher to come back, however, so the elf was compelled to return. The story Kreacher told was the last straw and Regulus became determined to destroy Voldemort.
From arrogant hints the Dark Lord had dropped, Regulus figured out what the locket was. Regulus ordered the elf to return to the island with him, substituted a fake horcrux for the real one, then told Kreacher to take the locket and destroy it. Regulus, who had drunk the potion in the stone basin to get to the locket, was attacked and drowned by the Inferi in the underground lake (DH10). He had forbidden Kreacher to tell anyone in the family what had happened, so they would be protected by not knowing. He is the infamous “R.A.B.”
Jo tells us "Regulus got in a little too deep. Like Draco. He was attracted to it [Voldemort's power], but the reality of what it meant was way too much to handle." (TLC)
Regulus = Latin for 'the little king;' the brightest star in the constellation Leo. Regulus was also the family name of Marcus Atilius Regulus, a Roman naval commander who was legendary for his heroic self- sacrifice (CE).
Arcturus = Latin for 'bear guardian;' the fourth brightest star in the night sky, whose rising and setting was thought by the Romans to foretell tempestuous weather.
Family in general: "The Noble and Most Ancient House of Black" dating to at least the Middle Ages (OP6); descendent of Phineas Nigellus Black, a Hogwarts Headmaster. Parents convinced that to be a 'Black' made you "practically royal." Black family members tend to be named for constellations.
Mother: Walburga Black (1925-1985); her portrait is permanently mounted in the hallway of the Black family home.
Father: Orion Black (1929-1979), who heavily fortified the family home at 12 Grimmauld Place, London.
Grandparents: Arcturus (1901-1991) and Melania (McMillan) Black, and Pollux (1912-1990) and Irma (Crabbe) Black. Both sets of Sirius's grandparents were direct descendents of Phineas Nigellus Black. One of his grandfathers (either Arcturus or Pollux) was awarded Order of Merlin 1st Class for "Services to the Ministry" (OP6).
Aunts, Uncles: Most notably Uncle Alphard, who left Sirius money, and Aunt Elladora who started the tradition of beheading house-elves when they were too old to carry tea trays. (See the Black Family Tree for more).
Cousins: Narcissa (Black) Malfoy, Andromeda (Black) Tonks, Bellatrix (Black) Lestrange. Arthur Weasley is a second cousin once removed. Distantly related to Molly Weasley.
Family motto: Toujours pur (French for "always pure") (OP6).
Family crest: is mentioned in OP6 but not described; however, Rowling drew it on her Black Family Tree. It shows a shield with hounds rampant emblazoned with a chevron, 2 five-pointed stars and a short sword.
Siblings: Sirius Black (c.1960-1996).
Location of childhood home: 12 Grimmauld Place, London.
Hogwarts: Slytherin c.1972; Seeker.