Regulus Corneas/Relationships

Pandora
Pandora persuaded Regulus to accompany her to attack Elior Forest alongside her. Also, Regulus saw Pandora as a higher ranked member of the Witch Cult, referring to her with "sama". But no matter how immense his respect for her was, Regulus was more than willing to shred her to pieces when she asked him to spare Petelgeuse's life.

Petelgeuse Romanee-Conti
Regulus was well acquainted with Petelgeuse as they were both fellow member of the Witch Cult and Sin Archbishops for a better part of a century. Regulus felt nothing but scorn and dislike towards Petelgeuse, referring to him as "abominable" and "scum" on many occasions. Petelgeuse entrusted Regulus to not bring Pandora to Elior Forest but he betrayed him and the village of elves. When Petelgeuse took in the Witch Factors of Sloth, Regulus mocked his incompatibility with the Witch Factors and expressed his disgust for his pitiful state, right before their battle began. During their fight, it's revealed that Regulus despised the fact Petelgeuse was considered the leader of the Witch Cult as stated he should have been the one entrusted with such an important seat.

A century ago, the two were also leaders of their respective Witch Cult factions; Regulus was the head of the radical faction while Petelgeuse commanded the moderate faction.

Lye Batenkaitos
Lye and Regulus besieged Crusch's and Rem's caravan in Arc 3 interlude. While Regulus was stalling Rem and Crusch, Lye took care of the knights that were accompanying the caravan. They both expressed dislike for each other, as seen when Regulus scolded Lye's uncontrollable hunger, and the latter snapped back at him, saying "they hate lecturing and they don't care about Regulus' views and ideals". That being said, they weren't hostile towards each other, despite the well known fact of Sin Archbishops fighting amongst each other.

In the anime adaptation of the Arc 3, Interlude 2, Regulus even helped Lye take care of Rem quicker by incapacitating her, giving Lye a chance to eat her name and memories.

Sirius
Regulus' and Sirius' relationship was extremely hostile. Regulus expressed deep disgust of her one-sided love towards Petelgeuse, even going as far as calling her worthless scum and a flawed being and didn't hesitate to attempt to kill her.

Natsuki Subaru
Subaru confronted him during his and Emilia's wedding in Priestella, which led to a battle that Reinhard joined in on and concluded with Regulus' death. However, Regulus and Subaru started off on a rather respectful note, but they soon started to strongly resent each other; Subaru's dislike stemmed from Regulus' abduction of Emilia while Regulus found fault in Subaru's talkativeness and disregarding of his opinions which, for him, meant Subaru was belittling his rights, therefore he felt he should have been punished accordingly.

Fortuna
Regulus came to Elior Forest in order to forcefully marry Fortuna, however, after seeing her attitude towards him and Pandora, he quickly abandoned the idea and set his mind on turning her to minced meat; which he would have done it it wasn't for Pandora removing his presence from the forest.

Emilia
Regulus and Emilia first met 100 years before Subaru's arrival, in the Elior Forest. Regulus would always remember her as that brat who had been crying and yelling nonstop when he went to retrieve his 79th wife.

After Fortuna's death, Regulus kept the 79th seat vacant for someone special. Upon laying eyes on Emilia in the Watergate City of Priestella, he set his mind on marrying her. At first, Emilia was very grateful for him saving her but soon she realized something was off. His way of speaking and his aura of death unsettled her. She soon made up her mind on saving his wives and defeating the Sin Archbishop. Regulus on the other hand thought quite fondly of her since she had a beautiful face, was pure, and was a virgin, which meant she checked all the boxes for a "perfect wife". Soon after Subaru and Reinhard's arrival however, he realized she talks too much and decided on killing her. He realized her true identity and her connection with the events of Elior Forest moments before he hit the ground, setting his mind on killing her no matter what it takes. Sadly, it was too late for him, and he drowned with only one wish; that Emilia would not get any satisfaction from his death.

His first wife
Ever since he was a small boy, Regulus always had his eyes set on one village girl. Not much is known about her, though Regulus saw her frequently and she was said to have not thought highly of him. Once he obtained the Witch Factor of Greed, he slaughtered her family, broke her mind and forced her to marry him, thus becoming his first of many wives. She never laughed, let alone enriched his mundane and monotone life with a smile but due to Regulus' warped and distorted view on the world, he didn't think much of it; to him, all that mattered was her beautiful face which was the reason he married her in the first place. Somewhere down the line she died in an unexplained way, but right before dying, she laughed for the first time. Regulus didn't take that laugh all too well, swearing he'll never be alone anymore. On top of that, he forbid his future wives to show any facial emotions; a rule which would result in their deaths, if broken.

She was also the only wife who Regulus made a grave for.

His wives
For well over 100 years, Regulus had been married 291 times. In order for his Lion's heart ability to work properly, he had been keeping them by his side all the time. He had been choosing his wives based on their faces. Accordingly, smiling, crying or any other form of facial expression that his wives weren't displaying at the time of their first encounter was strictly prohibited and would result in their deaths (a strong habit/rule he developed after his first wife's demise). Although he claimed he loved them all equally, he would not hesitate to kill them if he felt like his rights were belittled or if he wasn't satisfied with their faces. Throughout the years, he had killed 238 wives, leaving only 53 alive. It is also worth noting that some of his wives were just as corrupt and scummy as him. Some killed people, some were Witch Cultists, some enjoyed torturing people and animals, and some were greedy. One wife even sold her younger brother for a new dress.

Regulus thought love makes it so people are unable to be independent, so he decided on gathering a group of good-looking women who were virgins, as to not be betrayed and/or fall in love with them, resulting in him being independent.