Excalibur is a sentient, legendary sword who is widely regarded as the most powerful weapon in the world. He appears as a small, simplistic anthropomorphic creature with a stature comparable to that of a child. His facial features include large, bulbous eyes and a prominent, upturned snout, while his slender limbs and rabbit-like feet support a small white body. He is almost always seen wearing high-quality attire: a white top hat nearly as tall as himself, a similarly colored shirt with a frilled ruff, and a white jacket, though he wears nothing on his lower half. He never appears without a white cane, which is an extension of himself and transforms along with him. In his weapon form, Excalibur becomes a radiant golden broadsword, a double-edged blade with a precisely ornamented cruciform hilt engraved with regal inscriptions.

Excalibur is a Great Old One, a being whose very existence predates the demon weapon race and served as the inspiration for their creation. His soul wavelength is considered to be on a divine level, and his mere presence is reputed to be capable of driving ordinary people mad. As a member of the former Eight Reaper Legions, he is one of the most ancient and powerful entities in the world, though even his considerable power is said to be less than that of the Kishin Asura.

Despite his divine status and immense power, Excalibur's personality is famously insufferable. He is a profound narcissist, possessing an ego that surpasses even that of the highly arrogant Black Star. He is self-absorbed, has a short temper, and holds an extensive superiority complex, viewing virtually everyone else with contempt. His most frequent declaration is the word fool, which he uses to interrupt others and dismiss them. He is known for launching into long, nonsensical, and confusing rants about his supposed mythic exploits, claiming to have been a great performer, detective, and intellectual. He also has a habit of bursting into song on a whim. Excalibur demands that his meister follow a list of one thousand specific and absurd provisions, which govern nearly every aspect of their daily life. These requirements, combined with his grating mannerisms like sneezing fits or waving his cane mere millimeters from someone's face, make him nearly impossible to tolerate for extended periods.

Beneath this obnoxious exterior, however, hints of a more complex personality exist. In accounts of his time with King Arthur, he was portrayed as serious, honorable, and caring, once comforting his meister in a time of conflict. He has also revealed to others a past marked by frustration and a feeling that nothing could satisfy him, a period that ended only after he found happiness with a person named Cathy. He has demonstrated moments of complete seriousness, particularly with the god of Death, and has been willing to lend his power without his usual provisions in desperate situations, a complexity that suggests his insufferable demeanor may be a product of his nature as an elder god of anger and madness.

Excalibur's history stretches back centuries. He was once the weapon partner of King Arthur in the 12th century, helping him unite Britain and defeat Rome. He also claims to have resided on Baker Street with Sherlock Holmes, though by his own account, he was a largely incompetent detective who once had his partner Watson arrested for a murder committed by someone else. In the present day of the story, he resides in the Eternal Cave in the British Isles. His role in the main plot is often peripheral but significant. When first encountered by Black Star and Death the Kid, they find him so annoying that they immediately refuse to become his meisters. Later, a student named Hero the Brave successfully fulfills all of Excalibur's provisions and wields him for a time, using his overwhelming power to easily defeat top students from the Death Weapon Meister Academy. However, Hero eventually returns him to the cave, unable to bear his constant sneezing.

During a mission inside the Book of Eibon, Excalibur offers his teleportation abilities to the Spartoi team to help them reach their captured comrade. Liz Thompson volunteers to wield him, but her own amplified pride from the book's influence causes her to fling Excalibur back to his home rather than accept his help. Near the end of the story, Excalibur is present with the god of Death during the final battle against Asura. After the god of Death passes away, Excalibur promises to watch over his son, Death the Kid. When a guilt-ridden Kid returns to find his father gone, it is Excalibur who reminds him of his father's expectations, helping the new Shinigami find the strength to assume his role.

In terms of abilities, Excalibur is unmatched in raw power. Anyone who wields him is enveloped in Wings of Light, granting the ability of flight at high speeds. The wielder also gains the power of instantaneous teleportation, capable of moving at the speed of light and dodging even the fastest attacks. Each swing of the sword is said to slice through space itself. Excalibur is also capable of unleashing a powerful explosion of holy origin, an attack known as Hero the Atomic, which is strong enough to defeat multiple elite fighters in an instant. Despite his godly power, Excalibur can resonate with any meister, as his soul wavelength is universally compatible. The only barrier to wielding him is possessing the immense strength of will required to endure his personality long enough to accomplish anything meaningful, a feat that elevates those who manage it to the status of heroes and kings.