Sometime in the past, Ciel has had a traumatic experience with Baron Kelvin Renbourn. This led to him meeting his butler, Sebastian, and through unexplained circumstances, Ciel became a demon and gained immortality.
In Field Day!, Ciel competes in the Home-Run Contest and The Footrace, with his butler secretly aiding him in both games. For the Team Games, Ciel paired with Index of Sector SA. During the Whisper Hour, Ciel confided his secret to Index, and he demanded to know her true name, for he suspected what it was. Index merely told him the knowledge of her true name could destroy the world.
In The Horrorverse, Ciel invites Sandman to his manor in London, after Carol informs Ciel that Nolan is searching for Kelvin Renbourn. Nolan finds Ciel's manor and has Sebastian battle him. Sandman wins the battle, so Ciel orders Sebastian to be his partner as they infiltrate Noah's Ark Circus to find Kelvin. The two would eventually find Kelvin's manor and battle the man, and during this time, Ciel would locate the manor by tracking Sebastian. Kelvin was joyed to see Ciel again, but upon observing his blood-red eye and sensing the demonic darkness in his heart, Kelvin cries and pleads for the real Ciel to come back. Ciel kicks him off and Sebastian kills him.
Ciel then orders Sebastian to burn the building down, with Nolan insisting they need to help the children first. Ciel states that they're better off dead, and Nolan argues that a KND operative saves kids and brings hope. When Nolan calls Ciel a demon, Ciel thinks of his friends of Sector $, as well as Lizzy Midford, and he faints. Sebastian carries him back home, allowing Nolan to save the children before burning Kelvin's mansion.
In Sector $, Ciel is invited to Iceberg Gala by the Corporate Presidents For Children's Entertainment, who ask him to become a member. However, Ciel refused, and having invited his team to the party, they proceed to destroy the Hydra Weldar Gruntilda had brought.
Ciel is a well-proportioned 13-year-old boy with a skinny body, blackish-blue smooth hair, and a rich blue left eye (which turns scarlet when revealing his demon self). His right eye is always covered by an eyepatch, but it is actually a light-purple eye with a star symbol, the mark of his and Sebastian's contract. He wears noble attire and has an extensive wardrobe, and whenever he goes out, he wears a black top-hat.
"There is no such thing as true friendship. Friends are only valuable when they have something to contribute. That is why every Kids Next Door operative has to have a role in the team, otherwise they are useless. That's the only reason we value each other." -Ciel
Ciel typically speaks in a quiet, strict tone, and his accent is British. He is usually unhappy and keeps to himself, never letting his teammates know where he lives. He seems to have a slight attachment to his team, as he has a photo of them in his foyer. He has great fear and hatred of Kelvin Renbourn, and just hearing his voice triggers bad memories.
Ciel has demonstrated feats of strength and athleticism. He mostly relies on his butler, Sebastian for strength, for Sebastian will never refuse an order from him. Ciel is also very wealthy like his sectormates.
As a demon, Ciel can't swim, nor can he stomach mortal food. He is also vulnerable to lightbending and holy items, such as Index's habit.