Regulus's face had no eye-catching features, and he had golden eyes that were usually murky and disinterested but started to sparkle when he passionately engaged in his long monologues. His modestly handsome face wasn't particularly attractive or unattractive, while his hair was neither short nor long and without a distinct style, however, it was naturally white despite his youthful appearance. Regulus' white skin that lacked any semblance of a tan, coupled with his white hair and predominately white clothing, made him seem as though he was the epitome of the color white. In fact, the complete devoid of color exhibited by Regulus' physical appearance gave him the impression of a sickly person of which all color had drained away.

Regulus' primary attire primary piece of clothing was a pure-white greca-style overcoat that reached down to his ankles and had golden-edged cuffs, a set of five golden buttons running down the torso of his center front that usually ensured that the coat was closed above the hipline, and golden-highlighted edging. The inside of the coat was black and featured vertical golden stripes travelling down the entire coat. The most striking part of his coat was the opulent gold and black high-neck collar it sported that had the symbol for infinity inscribed on it, perhaps reflecting the nature of Regulus' Authority. Under his coat, he wore a long-sleeved deep-blue-colored dress shirt, white snow-white trousers, and a pair of white shoes, the latter two of which were both the same shade as his hair and overcoat. Usually, only the cuffs of the dress shirt could be seen as Regulus's long overcoat tended to obscure it almost completely. Regulus had a single deep-blue teardrop-shaped earring that was the same color as his dress shirt and dangled from his right earlobe thanks to a thin gold chain attaching it.

Regulus was talkative, and tended to describe himself as unselfish, despite representing Greed. He would often go on and on about trivial matters in the most incoherent ways, usually leaving the other party confused and/or unable to follow his thoughts, ideals and values. Due to his extremely prideful personality, however, he often forced his own selfishness on others and tried to justify it. If his opponents had said something that went against his opinions and beliefs, he would find unreasonable faults in them, gradually becoming angry while accusing them of "violating his rights", before attacking and killing them without mercy or second thought. Despite his self-proclaimed pacifistic nature, he would not hesitate to attack, harm or kill someone, as to him, the act of taking away what is his was the most abhorrent and disgusting thing a person could do. He often showed complete ignorance for other individuals' values and exercised extremely selfish behaviors. This is best showcased when Regulus became unreasonably infuriated at Rem when she prepared to defend herself after explicitly telling her that he abhors fighting, which contradicts the fact that he mercilessly slaughtered multiple convoys full of wounded people just minutes prior to Rem's battle with him and Lye Batenkaitos. Regulus is incredibly hypocritical and wouldn't hesitate to renege his own words and promises, giving further evidence of his sociopathic nature.

He was extremely prideful and viewed himself as a "divine being" or "the most satisfied existence in the world", which greatly hindered his assessment of people, no matter if they were his enemies or allies, to the point he would make light of the Sword Saint, mocking him and calling him "a moniker of some guy who knows of nothing but swinging a sword". He often pointed out his dislike for needlessly involving himself with others and vice versa, stating he would be perfectly happy if people never paid attention to him and/or bothered him. Despite that, he has been shown to have conversed with people without initiating a conflict with the opposite party, until he got his "rights violated", which instantly changed his mood for the worse and prompted him to take action against them. His hot-tempered personality often resulted in unneeded fights. However, due to his overwhelmingly powerful Authority of Greed, he always came out victorious and unharmed, even against ridiculously strong opponents such as the Vollachia's strongest Kurgan. Due to his long life as a Sin Archbishop without ever getting his life seriously threatened, he never matured intellectually or emotionally; he disclosed his secret to Subaru, who in turn was able to connect all the dots and unravel his formidable Authority, allowing Reinhard to deal the finishing blow. In his final moments, Regulus revealed how he hated being looked down upon or laughed at, going even as far as saying he became a Sin Archbishop just so he could protect himself from such people; whoever offended him or his wives would ultimately be slain for the transgression, no matter their acquaintanceship with him. Regulus never realized his "sins", even in the final moments leading to his demise. His view on the world was corrupt and distorted from the very beginning until the bitter end. His final cries were full of hatred, wishing for the whole of humanity to disappear forever.

Having been born with a corrupt and distorted mindset, Regulus seemingly did not perceive kindness, love and affection in the same way as a normal person would have; it was strongly suggested that he lacked empathy and was impulsive, narcissistic and sociopathic. He took everything as a personal assault and critique, never developed a "human side", which in turn made him feel like everyone was against him, pitying and loathing his very existence. With time, Regulus himself seemingly developed a paranoid personality which completely dulled his perception of the world around him. After receiving the Witch Factor of Greed, Regulus gained an extreme form of escapism; he slaughtered everyone who pitied him, destroyed everything that bothered him and protected only the things he deemed important and necessary for his ideal and dream life. With that, Regulus was given an opportunity to be able to escape the dull, depressing and detested world he was brought into, and create a new world with himself at the top and surrounded by his subjects/wives; his "cherished kingdom". He spent the following century trying to fulfill his Greed, which he never achieved.

More than 100 years ago, Regulus used to live with his family in a village of a country outside the Four Great Nations. His father was an alcoholic yet tried to be a good father, his mother complained about bad earnings but apologized for it occasionally, and his two brothers who were sometimes mean to him but gave him their share of their meal if he accidentally spilled his. At an unspecified point in his life, Regulus was chosen as a Sin Archbishop and obtained the Witch Factor of Greed. In accordance with his deepest desires, he developed the overpowered Authority of Greed, which he used in the following century of his life. Regulus subsequently killed his entire family, then the villagers who looked down on him, followed by the people from the nearby town who pushed his family into such peril, and finally, his entire home country. He broke the mind of his childhood love, slaughtered her family, and forced her to marry him, thus compelling her to become his wife. Only after having committed such atrocities did Regulus finally see a way to live: free of all worries, not needing anything as he considered himself satisfied as he was. However, she eventually committed suicide, sneering in her last moments. Happening across this, Regulus buried and made a grave for her, and began to search for wives after her, forcing them to remain expressionless and never smile like his childhood friend did.