Echidna[]
Years before the Great Calamity, Echidna encountered young Roswaal who was unable to utilize his insane magical talent and potential and was being treated as a sickly nuisance by everyone who encountered him as back then, magic was very much misunderstood and mages weren't as renowned as they are now (with few exceptions). Echidna sensed his ability and potentially devastating power and decided to take him under her mentorship, teaching him how to properly use magic and opening Roswaal's eyes, allowing him to recognize that his body isn't a "danger" but a "gift", allowing him to prevail and learn how to effectively use all six magical elements. As a result, Roswaal harbored intense and deep feelings for her, which were unfortunately very one-sided as Echidna is unable to comprehend human emotions. She was the most important person to him. After her death, Roswaal became obsessed with reviving her, even being prepared to give his life in order to achieve his desired future, kill the Dragon and cross paths with Echidna once more.
Beatrice[]
Both of them are acquainted with each other for more than 400 years, as they were both under Echidna's mentorship and protection. Before Echidna's demise, Beatrice formed a pact with her and vowed to protect the Forbidden Library located in Roswaal's Mansion. After Echidna's demise, Beatrice shut herself into the library, spending all of her time in there. Despite that, Roswaal still kept on visiting her, even after the original Roswaal A Mathers "died". He never confessed to her that he was the original Roswaal inhabiting the bodies of his descendants so their relationship became one between strangers. Tappei later confirmed Roswaal loved Beatrice and deeply cherished her and the relationship they had, despite Beatrice distancing herself. Although as expected, he would sacrifice her for his grand goal, despite the fondness he had of her. His only regret is how cold her attitude is towards him.
Ram[]
Through his Book of Wisdom, Roswaal prematurely learned of the incoming Witch Cult attack on the hidden Oni Village, however, he chose to follow the Book's writ and not prevent it. Further acting on the Book's instructions, Roswaal proceeded to save Ram and Rem, effectively making them the only two survivors of that village. When Ram learned the truth, she was enraged and swore to take revenge in the name of her wiped-out clan. However, things did not turn out the way she expected them to and her anger eventually subsided. Roswaal and Ram formed a contract, which binded Ram to help Roswaal with his schemes of killing the Dragon. In exchange, she could do whatever she desired with him once he had achieved his goal or whenever he surmised that the prophecy in his copy of the Book of Wisdom would not come to pass in the current world.
Roswaal took in Ram and Rem under his wing and is shown to deeply care for their well-beings. He oftentimes used Ram as an informant on matters, which showed the level of their mutual trust. Despite this, he has no qualms on sacrificing her to meet his goals as shown in one of Arc 4's failed loops, where he kills Garfiel, despite Ram being in the way. It is worth noting however that he knew he was going to die, and that Subaru would loop to bring her back anyway.
Throughout the years, Ram's hate for Roswaal slowly dissipated and turned into feelings of affection and love. During Arc 4's climax, Ram ended up confessing her feelings for him, which shook him to his core. Since his feelings hadn't change for the last 400 years, he assumed that Ram's hadn't changed either. However, it is shown that he is emotionally attached to her.
Rem[]
Roswaal took Rem and Ram under his wing and was shown to deeply care for them, as seen when Rem died of unknown causes and he immediately suspected that Subaru is responsible (he was even prepared to kill him, much like Ram). While she is dutiful, unlike Ram, Rem has no strong loyalty towards Roswaal. Her obedience is borne more from his necessity to Ram's condition and being the owner of their new home she wants to protect.
Garfiel Tinsel[]
Garfiel never trusted Roswaal, and his actions during the business in the Sanctuary only made things worse. However, Garfiel recognizes Roswaal’s strengths and knows that the camp needs him. Still, anytime Roswaal was present, he kept his guard up and actively made visible his spite.
On the contrary, Roswaal seems not to have any negative feelings towards Garfiel, and he is even willing to give him guidance in combat, such as in the case of confronting Olbart Dunkelkenn in Lupugana.
Emilia[]
Emilia's benefactor and backer in her bid to acquire the throne. He's considered an odd lord, especially due to his fondness for demi-humans like her to the point where he would hire them. In truth, he is only actually using her to control Subaru and coerce him into using Return by Death to fulfill his ambition of reviving his late master through the wits of his Gospel. To do so, he needs a guaranteed way to put Emilia on the throne because he has no faith in Emilia's resolve to overcome her past and deeply-rooted hatred of the Witch of Envy. However, with the encouragement and support of Subaru and several others, Emilia shatters his expectations by passing all 3 of Echidna's trials. He is then forced into backing her earnestly.
Natsuki Subaru[]
When they first met, Roswaal appeared to be genuinely grateful for Subaru rescuing Emilia's insignia, even offering him anything he wants as gratitude. He appears to be rather friendly to the young boy. However, Roswaal is more than prepared to kill Subaru in one of the loops, connecting him to be the cause of Rem's unknown death.
In Arc 4, their relationship became rather antagonistic (i.e. giving Subaru half-truths or intentionally throwing monkey wrenches in the latter's plan). Roswaal becomes aware of Subaru's "Return by Death" and later admits that he's molding Subaru to become like himself (acting on his own desires rather than waste time saving someone's life). By the climax of said Arc, Roswaal and Subaru have placed a bet, which he lost (although he will force Subaru to use Return By Death, should he make a mistake).
Ryuzu Meyer[]
Ryuzu and Roswaal were classmates being taught by Echidna. They were also very close friends.
Serena Dracroy[]
Having once assisted Serena in the past, the two became friends. When the two met again in Arc 7, she even made a jab at his nickname, the Demi-Human Lover, as a joke. However, it should be noted that he is not afraid of coming to blows if she stands in the way of his goals (i.e. Subaru being trapped in Vollachia). Fortunately, there was no need for this to happen.
Olbart Dunkelkenn[]
Although they would meet for the first time during the Decisive Battle for Lupugana, Roswaal instantly recognized who he was and the threat level that he posed. Likewise, Olbart was wary of him and found him to be difficult to deal with.
Balleroy Temeglyph[]
During a meeting in the past regarding how to handle Vollachia, Roswaal recognized Balleroy as one of the biggest threats and sought to create countermeasures against him. However, Balleroy would end up dying several months before the start of the story. Ironically, however, Roswaal would get a chance to confront the man during the Great Disaster, where the Magical Sharpshooter would be brought back to life as a zombie. During their clash, Roswaal stated that under ordinary circumstances, Roswaal would lose to Balleroy. Conversely, Balleroy despised the way Roswaal handled their fight, using Medium, as a practical meat shield, and throwing out numerous attacks with various targets simply to divert the man’s attention.
Hector[]
Hector and Roswaal used to share an extremely hostile relationship. Roughly 400 years ago, Hector pursued Echidna and located her at her hideout in the Sanctuary. Roswaal faced off against him, trying to fend him off and protect the Sanctuary but he failed miserably, being one step away from death. At the last second, Echidna came to the rescue, saving Roswaal's life but at a cost of his ego being utterly shattered. In the following years after his defeat, Roswaal started wearing the same clothes and talk in the same manner as Hector, in order to not forget the humiliation he suffered all those years ago.
Volcanica[]
Roswaal’s primary goal is the death of the dragon, though it is for unknown reason. Considering Volcanica ordered the Mathers family to handle the Witch of Greed’s remains, he may have perhaps met a young Altair once in the past. However, this remains to be seen.
Sphinx[]
When Sphinx made her presence known during the Demi-Human War, Roswaal, who at the time was in the body of Roswaal J Mathers, actively sought her out, learning martial arts solely to counter the Witch’s magical prowess. Though Roswaal would not land the finishing blow on Sphinx, he was led to believe that the failure of Echidna was dead.
In Arc 8, Roswaal would encounter the zombie of Sphinx while intending to study the bodies of the zombies with Beatrice. Naturally, his hatred for the Witch started to boil within him, and when Sphinx would not answer him, he quickly engaged in combat against her, destroying this body alongside Beatrice and Garfiel.