Crowley Eusford is publicly known as a vampire of the Thirteenth Progenitor rank, but his true power is far greater, being equivalent to a Seventh Progenitor. This discrepancy is due to the unusual circumstances of his creation. He was once a human knight and a member of the Crusades in the 13th century, hailing from a noble family. After enduring a traumatic event where his remaining comrades were killed by vampires, he returned home and withdrew from active combat, instead spending his time training young men in swordsmanship. His transformation into a vampire was orchestrated by the Seventh Progenitor, Ferid Bathory, who used the blood of the original Second Progenitor, Saito, to turn him, rather than his own. Because creating a vampire with another's blood is strictly forbidden, this secret must be kept hidden to avoid a horrific punishment.

Crowley possesses a calm, composed, and leisurely demeanor, often greeting situations with a confident smile. He is driven by a strong sense of curiosity and a desire to avoid boredom, which is a primary reason he aligns himself with the unpredictable and scheming Ferid Bathory. Despite his immense power, he rarely reveals its full extent, preferring to observe and assess situations with an analytical mind. He once noted that as a human he killed more people than he has as a vampire, showing a pragmatic and somewhat detached view of violence. He dislikes drinking the blood of children and is often more interested in the evolving strength of humans and the mysterious plans being set in motion around him.

In the story's Battle in Nagoya arc, Crowley plays a significant role as one of the noble vampires occupying Nagoya. He commands his two loyal and beautiful subordinates, the female vampires Chess Belle and Horn Skuld, both of the Seventeenth Progenitor rank. Together, they decimate a unit of the Japanese Imperial Demon Army, taking many survivors hostage to lure the commander, Lieutenant Colonel Guren Ichinose. Ultimately, Crowley personally captures Guren and holds him prisoner, displaying overwhelming strength against the efforts of Guren’s squad to rescue him. During the ensuing battle, he demonstrates his extraordinary capabilities by effortlessly blocking a sniper shot from Yoichi Saotome that other nobles could not evade, and fighting against a demon-powered Yuichiro Hyakuya without being pushed to his limit.

His most important relationship is with Ferid Bathory. Though Ferid is responsible for the destruction of his human life and the death of his friends, Crowley eventually let go of his anger and chose to ally with him, finding the centuries of existence with Ferid to be interesting and devoid of boredom. He also holds a position of respect from his two attendants, who are fiercely loyal and attentive to his commands. Over the course of the series, Crowley continues to involve himself in major events, developing from a seemingly idle noble into an active participant. He travels with Ferid and Yuichiro's group to Osaka, where he defeats a Tenth Progenitor in a single strike, finally admitting to his hidden power level. He later survives being decapitated by a Fifth Progenitor, reattaching his head and continuing to fight. As a vampire, Crowley possesses fangs, pointed ears, and red eyes. He is notably tall and muscular, standing around 190 centimeters, with a distinctive hairstyle featuring red bangs and the rest of his dark hair tied into a long braid. He carries a一级武装 or first-class military sword that can sprout thorns to pierce his hand and absorb his blood, dramatically increasing its speed and power. His true strength, derived from the blood of the Second Progenitor, allows him to perform feats far beyond his official rank, such as breaking walls with his bare hands and generating powerful shockwaves with his sword.