As the head butler, Sebas was assigned to attend to Ainz's needs and is always at his side before he was sent away on a mission. Sebas is extremely loyal to Ainz as he is the last of the Supreme Being to the point where he would kill Tuare without hesitation if Ainz ordered him to. But Sebas would sometimes scold Ainz when he went out on his own.
Touch Me is the creator of Sebas and like all other created NPC, Sebas is extremely loyal to his creator. It was also stated that if he had to pledge his loyalty to only one among the Forty-One Supreme Beings, Sebas would choose Touch Me without hesitation. After the transfer of Nazarick to the New World, he has acquired a lot of similar traits as his creator; likely due to his being a blank slate, with no backstory programmed by Touch Me. He tries to live up to his creator's honor and kindness to those less powerful than him.
Albedo is Sebas' colleague. As Albedo is Leader of the Floor Guardians, while Sebas leads the Pleiades, she has no authority over him except in matters pertaining to the upkeep of Nazarick or if Ainz says so. They accompany and serve Ainz in his duties in Nazarick. Albedo said to Sebas that if any situation arises, he should come back and report to her, especially if it's anything related to Ainz.
They worked together in their mission to capture humans who know how to use martial arts. Sebas thought that someone like Demiurge would be better suited for the task, something that made Shalltear very angry. While he doesn't like the way Demiurge treats humans, he's fine with Shalltear's evil personality. Curious of Shalltear bickering relation to Aura, the vampire guardian revealed their respective creators are actually siblings. This humbled Sebas, along with Solution and the Vampire Brides.
Sebas Tian and Demiurge don´t get along, although they do seem to understand and respect each other's abilities. Sebas dislikes Demiurge's cruel treatment of humans and finds him irritating to speak to. Even so, Sebas felt that Demiurge would be a better choice for capturing humans than Shalltear on their mission to research martial arts, much to Shalltear's disdain. It is believed that the reason for the friction in their relationship is due to their own creators, Touch Me and Ulbert Alain Odle, who didn't get along either but were shown to have respected each other.
Cocytus is Sebas's colleague. He was ready to fight and kill Sebas if he were to be revealed as a traitor. However, as Sebas' killing intent and attack on Tuare were genuine, Cocytus blocked the attack confirming the butler's loyalties; this proved Sebas loyal and their master Ainz to allow Tuare to stay. As both NPCs have Positive Karma ratings, both have honorable character tendencies, such as sparing those which earn their respect- i.e. the Lizardmen, or Brain and Climb. It is also worth noting that Touch Me (creator of Sebas) and Warrior Takemikazuchi (creator of Cocytus) were shown to be rather close, as they shared a friendly rivalry and spared often.
Aura Bella is Sebas's colleague. Not much interaction is seen of the Dark Elf Twins with Sebas, but they interact when their tasks coordinate, allowing for the completion of a mission by Ainz.
Mare Bello is Sebas's colleague. Not much interaction is seen of the Dark Elf Twins with Sebas, but they interact when their tasks coordinate, allowing for the completion of a mission by Ainz.
Victim is Sebas's colleague. He was ready to fight and kill Sebas if he were to be revealed as a traitor.
She is Sebas' successor as leader of the Pleiades. They have a positive co-worker relationship, likely due to their creators & positive karma rankings. Following Sebas' removal from the Pleiades by Ainz, Yuri remains his friend and helps in the maid training for Tuare under Sebas' care.
Lupusregina serves as Yuri and Sebas' subordinate.
Narberal serves as Yuri and Sebas' subordinate.
CZ Delta serves as Yuri and Sebas' subordinate.
As Sebas is her superior, Solution is loyal to him and would even apologize after acting rude to him although they know it was an act. However, Solution's loyalty towards the Supreme Beings is greater as she would even go against Sebas' order and report Tuare's existence to Ainz.
Entoma serves as Yuri and Sebas' subordinate.
After Sebas selflessly rescued Tuare from her horrible life as a brothel slave and having been the first person to treat her as a human being, instead of an object or a toy, the young woman eventually became smitten with the aged butler. This affection towards him is to the point of willingly braving the potential dangers of Nazarick just to be with him. He blushes when teased by Pestonya Shortcake Wanko on his relationship with Tuare, showing that he has feelings for Tuare. Despite his feelings, he is strict with Tuare in regards to her training to become a maid of Nazarick. For that reason, he wasn't willing to let her take a break until heeding the words from Yuri about her creator's words.
Staffan made Sebas Tian's acquaintance when the government official attempted to intimidate Sebas into paying a hefty fine for a false charge of slave trading. Sebas compared him to a pig to slaughter when the fat man turned red as he tries to justify his actions. During the raid on the brothel, Sebas Tian found Staffan naked beating a defenseless woman. Disgusted by his actions, the butler repeatedly backhanded the fat man across his face. After his ignominious flight from the Iron Butler, he tried basic pleading for his life, confirming to Sebas what it means to be unworthy to live, and kicked him full force in the abdomen, which ruptured his internal organs and left him to die in agony. He was the first 'truly disgusting' human Sebas ever came across in the New World.
Sebas admired Climb's determination to grow stronger despite his limitations and thus agreed to train him. He first teaches the young man how to overcome a powerful killing intent, and later offers mentor-like attention to teaching Climb as he fights off an assassin. He tells him that those who fight for something greater will always be stronger than those that only fight for themselves. During Tuare's rescue from Six Arms, Climb had developed a new Martial Art exclusive to himself which allowed him to consciously control his adrenal response for a momentary boost of strength.
Sebas thinks Brain can hold his own, unlike Climb who is still quite weak in comparison. After witnessing Climb's withstanding Sebas' killing aura, Brain learned a humbling lesson of strength from others. While highly skilled physically, Brain learns the importance of inner strength which allows him a swift victory over Succulent. He holds Sebas' words in high regard, viewing him as a powerful & honorable individual. Also acknowledging his strength to be on a similar yet weaker level to Shalltear Bloodfallen.