Clavis serves as the elevator operator within the arbiter system and also acts as a personal valet to Nona, a high-ranking arbiter. His primary function is to transport arbiters and the souls of the deceased, known as guests, between the various floors of the tower that houses the judgment bars like Quindecim. He is not an arbiter himself, but he is not human, occupying a unique supporting role within the afterlife's infrastructure.

Visually, Clavis is a young man with a striking and unconventional appearance. He is almost never seen without a serene and slightly mysterious smile on his face, and his eyes are perpetually closed or obscured, meaning he never truly shows them. He has spiky green hair with a distinct yellow streak on his left bangs, and the back of his head features a red undercut visible on both sides. His look is further accentuated by piercings on his right ear and nose. His uniform reflects his formal duties, consisting of a light brown blazer-like coat with dark brown collar and cuffs, worn over a white shirt and a red tie. He completes this outfit with gloves, brown pants, and black leather shoes.

Clavis's personality is a blend of professionalism and playful mischief. He carries out his duties with a consistently amiable and formal demeanor, always serving with a smile. Despite this polite exterior, he is known to enjoy teasing others, particularly the hot-headed arbiter Ginti, often provoking him with lighthearted jabs. He is a composed presence, even when cleaning up messes after conflicts, and enjoys a good drink. Underneath his cheerful surface, he is deeply loyal, especially to Nona, and can be a steadfast companion in moments of quiet empathy, such as when he stays by Ginti's side after a personal loss.

While he does not participate directly in the judgment of souls, Clavis's role is crucial to the smooth operation of the afterlife system. He is first seen transporting Nona and the mysterious black-haired woman to Quindecim. He later breaks up and cleans up after a physical fight between Arbiters Decim and Ginti, teasing the latter for his defeat. In a significant act of trust, Nona allows him to be aware that Decim, a traditional arbiter, has been implanted with human emotions, a secret she has hidden from the rest of the system. This knowledge makes him an unwitting pawn when the higher authority Oculus, suspecting Nona's scheme, summons Clavis and absorbs his memories to uncover her plan, an event Clavis remains completely oblivious to. Demonstrating his discretion and the trust placed in him, Clavis hesitantly but obediently grants Decim access to the lowest, most restricted level of the tower—the place where discarded dummies are disposed of—using a special key.

Clavis maintains key relationships with several main characters. His relationship with Nona is one of friendly trust and unwavering loyalty, serving as her dedicated attendant and confidant. With Decim, he is consistently polite and respectful, offering support when needed, such as giving him a day off after a stressful event and facilitating his secret request to visit the lowest floor. His relationship with Ginti is more complex, characterized by playful teasing and a deeper, unspoken bond; Clavis is shown to appreciate Ginti despite his abrasive personality, and in the end, he seeks out the red-haired arbiter to keep him company.

Notable abilities possessed by Clavis include the power to induce sleep in others with a simple touch. He demonstrates this when he effortlessly puts the aggressive Ginti to sleep after a fight. His name is derived from the Latin word for "key," which is a direct reference to his function as an elevator operator who controls access to the different floors of the tower. Throughout the series, he is shown to have an appreciation for music, seen genuinely enjoying an impromptu concert held in one of the bars.