Sebas Tian serves as head butler of the Great Tomb of Nazarick, personally created by the Supreme Being Touch Me. He commands the Pleiades combat unit and operates outside the Floor Guardians' chain of command despite matching their combat strength. His original purpose was to serve as Nazarick's final defensive line alongside the Pleiades on the 9th Floor, stalling intruders to buy time for guild members preparing a last stand in the Throne Room.

He presents as an elderly human with white hair, a beard, sharp eyes, and a dignified bearing, consistently attired in a traditional black butler uniform. This human guise facilitates reconnaissance missions in human settlements, a duty often shared with Solution Epsilon. His true nature is that of a dragonoid; revealing this form drastically boosts his combat power, enabling him to potentially overwhelm powerful guardians like Albedo and Cocytus.

His personality bears strong influence from creator Touch Me, emphasizing personal justice and protecting the weak—a stark contrast to most Nazarick denizens who despise humans. He firmly believes the strong bear responsibility to aid the vulnerable, actively assisting humans like Tuareninya Veyron, whom he saved from brutal enslavement. Yet, he exhibits utter disgust towards cruel or morally corrupt humans, dispatching such individuals without hesitation if they oppose Nazarick or embody depravity. Despite his compassion, loyalty to Nazarick and the Supreme Beings remains absolute. When his loyalty was tested over Tuare, he prepared to kill her upon perceiving orders from Ainz, prioritizing Nazarick above personal attachments. This duality creates internal conflict, sometimes causing him to question actions conflicting with his master's directives, a trait potentially inherited from his creator.

His capabilities rank him among Nazarick's four mightiest warriors. Specializing in unarmed combat as a Monk and Ki Master, he channels ki to enhance strikes beyond blade sharpness, heal non-poison/disease injuries, induce unconsciousness, or harden his skin. Passive abilities include sensing life signals (ki) and gauging enemy strength levels, unless concealed by skills or magic. He possesses immense physical power: effortlessly tearing steel doors from hinges, intercepting multiple thrown daggers in under a second, and delivering strikes that launch armored foes with lethal force. His durability allows him to withstand potent attacks, including Zero's ultimate martial art, unscathed. A tactical rock-paper-scissors dynamic exists among top warriors: Cocytus holds an advantage against Albedo, Sebas against Cocytus, and Albedo against Sebas.

Following Nazarick's transportation to the New World, initial duties involved close reconnaissance per Momonga's orders, seeking peaceful contact with intelligent life near the tomb. He later gathered intelligence in the Re-Estize Kingdom's capital, meticulously documenting requests and gossip for Nazarick reports while restraining Solution from indiscriminate harm to humans. Post-Sorcerer Kingdom founding, duties shifted to E-Rantel. Key missions included investigating Nazarick's surroundings, managing maid schedules under Ainz's rest policy, and aiding Shalltear Bloodfallen in capturing martial artists, though he considered Demiurge better suited. Rescuing Tuare from an Eight Fingers brothel triggered major events, leading to clashes with Six Arms and a subsequent loyalty test by leadership. During the raid, witnessing corrupt official Staffan Heivish assaulting a woman cemented his revulsion; he killed Heivish via a gut-rupturing kick.

Relationships within Nazarick are complex. He holds profound reverence for Touch Me, likely choosing him above other Supreme Beings if forced. Serving Ainz Ooal Gown as head butler demands constant attendance and deep loyalty, though he privately disapproves of Ainz venturing unguarded. Friction exists with Demiurge due to opposing philosophies mirroring their creators' (Touch Me and Ulbert Alain Odle) contentious relationship, breeding mutual irritation despite professional respect. He maintains positive rapport with honorable guardians like Cocytus, united by positive karma. As Pleiades leader, he commands respect, though Solution Epsilon reported Tuare to Ainz over his orders. Among humans, he admires Climb's determination, mentoring him to overcome killing intent and grasp martial principles. He views Brain Unglaus as competent but taught him inner resolve's importance. Deep care developed for Tuare, evident in blushing when teased about their relationship and strict training to integrate her as a Nazarick maid, though duty supersedes personal feelings.