Regulus Black

Regulus Arcturus Black (b. 17 August 1961) is a wizard and a former Death Eater. He is the younger brother of Sirius Black, and currently a fugitive.

Early Childhood
Regulus grew up, the youngest child in the Black family. For years he and his brother would clash in opinions, but still cared for one another. As Sirius became more rebellious towards his family, Regulus behaved. Regulus was sorted into Slytherin and he became close friends with Barty Crouch. Jr. When Sirius left home, Regulus found Sirius's notes on becoming an Animagus, and he and Barty secretly became Animagi, themselves, with Regulus as a cat, and Barty as a weasel.

First Wizarding War
When the first Wizarding War broke out, Regulus joined the Death Eaters, though he never had the will, in that time, to perform any Unforgivable Curses.

Betraying Voldemort and Imprisonment
Disenchanted upon seeing his House Elf, Kreacher, in such a ruined state testing the Dark Lord's defenses on a horcrux, Regulus chose to betray him. He went to the caves to find one of the horcrux, and was severely weakened by the defenses. But thanks to Kreacher's magic and Barty's support, he was able to heal. He told Barty that he was cursed because of a trap set by the Auror's, but Regulus accidentally gave away his true allegiance when Barty saw the look of excitement on Black's face when he said Voldemort was defeated. Shortly after, Barty, and the Lestrange's went after and tortured the Longbottoms and Regulus arrived to try and stop them. When they were arrested by the Aurors, the culprits pinned most of the blame on Regulus and they were all sent to Azkaban.

Gideon and Fabian Prewett would be the ones to escort Regulus and the other prisoners during the trials and would note that it seemed as if Regulus and Barty looked like they were duelling when they were arrested.

Regulus would feel guilt over his friend's imprisonment and apparent death. But one day, he saw in the newspaper a photo of the Weasley family and recognized the weasel as Crouch and broke out of the prison to find his former friend.

On the Run
After Regulus Black broke out, he spent the majority of his time as a cat. He became Ginny Weasley's cat Llewelyn,

Regulus Black escaped the Hogwarts grounds before the Second Wizarding War with the newly freed Buckbeak. In Harry Potter and the Half-Blood Prince, Buckbeak is seen at Steorran Castle, where Regulus has ensured the Hippogriff's safety on their family grounds.

Personality
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Trivia

 * Regulus' name and zodiac sign coincide with each other. In a letter to Harry in Goblet of Fire, Regulus mentions that Walburga gifted Sirius his penchant for bad puns.
 * Regulus' cat name - while not his choice - is Llewelyn, which is Welsh for "Like a lion".
 * His animagus form is a black cat with a white mark across its chest - a likeness to the small scar Regulus bears after a particularly bad fight he had with Sirius as a child.
 * While he hasn't said so himself directly, Sirius has mentioned that Regulus is fond of weddings a few times across the series.