Cocytus considers himself the 'sword' of Ainz Ooal Gown, an object that does as he is told without ever questioning his master. When Demiurge brings up that they could serve Ainz's possible future heir, he gets excited and hopes that he gets to be called 'uncle'. Also, when Ainz sends a Death Knight to ask the guardians what they want, Cocytus asks that Ainz produce an heir and hopes to teach the "young master" swordsmanship, but due to Ainz being a magic caster, he thinks that the heir will be an outstanding magic caster, so he settles for protecting the "young master" and even daydreams about it. Cocytus also greatly fears that Ainz will abandon them due to him disappointing him with his failure in conquering the Lizardmen and is greatly relieved after learning that Ainz does not blame nor is he angry at him.
Warrior Takemikazuchi is Cocytus's creator and the previous wielder of the God Slaying Emperor Blade. Cocytus holds this blade to be one of his prized possessions so as to keep the memory of his creator close. Cocytus also shares his creator's knack in creating suitable names.
Due to Cocytus' simple-minded thinking, Cocytus becomes the second guardian to fail a direct order from Ainz, which results in a brief scolding by Albedo. However, Cocytus made it clear that he will not stand by while someone shows disrespect toward his master, Ainz, even if it was the Oversee Guardian. Other than that, they are shown to be on good terms with each other.
Cocytus is one of Shalltear's colleagues. Like her, he has experienced the taste of failure, so they both share the same urge of redeeming themselves in front of Ainz.
As a fellow Floor Guardian, Demiurge is a colleague and, at the same time, a rival. Despite their rivalry for Ainz's attention and praise, they have a stable work-relationship. They are civil with one another to be considered drinking partners at Sous-chef's bar on the 9th Floor of Nazarick. He respects Demiurge's intelligence and confides in him on deciphering Ainz's plans.
Aura is Cocytus's colleague. Cocytus doesn't like it when Aura says that he's nude because it makes him sound like a pervert.
Mare is Cocytus's colleague.
Victim is Cocytus's colleague. They do not see each other often. Despite this, they maintain a cordial and respectful relationship.
Cocytus is on good terms with Kyouhukou.
Cocytus is on good terms with Gashokukochuuou.
Sebas is Cocytus's colleague and one of the few good alignment NPCs of Nazarick. Although he is ready to fight and kill Sebas if he were to be revealed as a traitor, he doesn't show any ill intention against the butler after his innocence is determined. It's unknown if the two of them share the same friendship and rivalry as their creators.
Cocytus is a superior of Yuri and the two are not seen interacting.
Cocytus is a superior of Lupusregina and the two are not seen interacting.
Cocytus and Narberal are close to each other and get along unexpectedly well due to their creators being close friends.
Cocytus is a superior to CZ2I28 and the two are not seen interacting.
Cocytus is a superior to Solution and the two are not seen interacting.
Entoma is sent by Ainz to accompany and observe Cocytus in the war against the lizardmen. Cocytus tries to read her expression but gives up since her face is just a mask used to hide her true identity. He even remembers the words of his friend, Kyouhukou, that Entoma is the scariest one of the Familiar Eaters.
For a short time, Iguva=41 was Cocytus's subordinate. Despite being under him, Cocytus felt guilty for losing him in battle to the lizardmen, since he was a personal creation of Ainz Ooal Gown.
Cocytus respects Zaryusu for his fighting spirit and ingenuity in combat. Cocytus petitions Ainz Ooal Gown to resurrect Zaryusu as he believes it would be a pity to condemn such an admirable warrior to oblivion. He is currently overseeing Zaryusu's training and is always on the lookout to provide better equipment for him and all of the lizardmen that are under his care.
After the conquest of the Great Lake, Zenberu becomes Cocytus' subordinate. They have a good relationship, though Cocytus is dismayed at the lack of proper manners Zenberu displays when talking to his master, Ainz Ooal Gown. He would have killed him on the spot for his insolence had it not been for Ainz's intervention.
Cocytus is one of Sous-chef's regulars at the bar.
Cocytus met the human when he led the Sorcerer Kingdom's army to wipe out the Royal Capital. Cocytus was met with a challenge by the human mercenary and per his warrior code was obliged to honor his request for a duel. Even though Cocytus could have easily won the duel by breaking his opponent's weapon, or even his limbs. But that was against the warrior's way and decided to give him a warrior's death. He had already sensed it when he saw this man standing on his own from afar, he knew it all the better when he stood face-to-face with him: this was a warrior who had made his resolve. Still he felt it was a shame that he had to kill Brain and wished he could have saved Brain's life and have him become loyal to his master. That is why he claimed Brain's weapon as his own, rather than use it as a headstone for Brain's grave. Despite it being of insignificant worth, it was still a piece of a worthy warrior. Cocytus also claimed Brain's remains as his spoils to honor the fallen swordsman.