Regulus

Regulus is Sairaorg's Pawn, and the spirit residing in the Longinus Regulus Nemea.

Appearance
A huge lion with golden fur and is known as the Nemean Lion. It is four to five meters tall, and has a jewel on its forehead.

Personality
Regulus is extremely loyal to Sairaorg, calling him "Master" and constantly tries to help Sairaorg by all means. He is shown to be an observant fighter.

History
According to Azazel, Regulus is the Nemean Lion, which the God from the Bible caught and sealed inside the Longinus Regulus Nemea. After its previous wielder died, Regulus somehow materialized himself and killed the group of people who murdered his possessor. It was also at that exact time that Regulus met Sairaorg who tamed him and reincarnated Regulus into a Devil through 7 Pawn Pieces.

The Heroic Oppai Dragon
Regulus appears in Volume 10, during Rias and Sairaorg's Rating Game. He faced Rias one-on-one and defeated her, telling Issei to use the Phoenix Tears if he wants to save her from retiring from blood loss. He then asked Sairaorg to wear him. While intially refusing, Sairaorg eventually agreed after Issei's persuasion to fight at full strength. The two of them begin to merge to attain the Balance Breaker and proceed to fight Issei.

He reappears in Volume 12 alongside his master but did not do anything.

Powers & Abilities
Promotion: As a Pawn, Regulus can use promotion to temporarily gain the traits of a Queen, Rook, Knight or Bishop.

Immense Power: Regulus possesses an immense amount of power. It is said that upon full mastery, the wielder of the Sacred Gear can split the Earth in half.

Trivia

 * In Greek mythology, the Nemean lion was a monster that terrorized the city of Nemea until it was slain by Heracles.
 * Regulus is the name of the brightest star in the Leo constellation.