Isley, also known as the White Silver King, is a character who holds a foundational place in the history of the world. He was originally the number one ranked warrior of the very first generation of Claymores, a time when the organization still created male warriors. As a Claymore, his power was said to be transcendent, with an enormous gap between him and the number two of his generation. At some point after many of his male comrades had awakened into monsters, Isley himself chose to awaken voluntarily, possibly with the intent to become strong enough to destroy the other Awakened Beings. However, upon awakening, his mind changed, and he instead became one of the first and most powerful of the Abyssal Ones, taking dominion over the northern lands. He remained an awakened being for over seventy-eight generations, yet he still retained much of his human memory, particularly his profound knowledge of swordsmanship.

In terms of personality, Isley is a figure of duality. On the surface, he often appears as a polite and cheerful young man with a calm demeanor. This exterior, however, masks a deeply calculating, cruel, and pragmatic nature. He is a master strategist who has no qualms about sacrificing his own army of awakened beings, including his loyal subordinates, to achieve a larger goal, demonstrating a callous disregard for the lives of those who serve him. He is arrogant and sees most other beings as beneath him, yet his pride is rooted in genuine, overwhelming power. Despite this coldness, Isley also displays a surprising and genuine capacity for human affection. He finds a sense of contentment and family with the young human boy Raki and the powerful awakened being Priscilla, a bond that becomes his most significant emotional development.

Isley's motivations are driven by ambition and a desire for absolute control. His primary goal is to conquer the entire continent and become the unrivaled power. To this end, he initiated the Northern Campaign, sending an army of thirty-one Awakened Beings to the north to draw out the Organization's warriors and his rival Abyssal Ones. His true aim was to dismantle the existing power structure. He orchestrated a brilliant, multi-pronged strategy, sending a portion of his forces to the east to occupy the Organization and another portion west to occupy Riful of the West, while he personally traveled south to eliminate Luciela of the South. He understood that an alliance between Riful and Luciela would be a threat, and by killing Luciela, he eliminated that possibility and secured his faction as the single most powerful on the continent.

The most pivotal relationship in Isley's life is with Priscilla. Publicly, it was believed that he defeated the rampaging Priscilla and made her his consort. In reality, when they first met, Priscilla nearly killed him, tearing off half of his torso before her mind inexplicably regressed to a childlike state. Seeing an opportunity, Isley swore allegiance to her, initially intending to use her as an unstoppable weapon to achieve his ambitions. Over time, however, his feelings evolved from mere utility to genuine affection, and he came to see her as family, taking care of her as one would a child. His relationship with Raki began when the boy wandered into the north. Finding that Priscilla was drawn to him, Isley took Raki in and saw in him a reflection of his own youth. He began training Raki in swordsmanship, and over seven years, this makeshift family became the most important thing to him. This is most powerfully demonstrated at the end of his life; when he was hunted relentlessly by the Abyss Feeders, he sent Raki and Priscilla away to protect them, choosing to face his death alone. Other key relationships include his rivalry with Riful of the West, an enemy whose power he respected but whose plans he consistently outmaneuvered. He was also the master of Rigardo, the Silver King of the North, who served as his loyal right hand.

Isley's role in the story is that of a primary antagonist and a major catalyst for conflict. His calculated decision to start the Northern Campaign directly leads to some of the series' most significant battles and character moments. After defeating Luciela, he remained the ruler of the south for seven years, a period of relative quiet during which he trained Raki. His development from a purely ambitious Abyssal One to a being who finds solace and happiness in a simple family life is his central arc. In his final moments, being slowly consumed by the Abyss Feeders, he does not think of his lost power or failed conquests, but instead wistfully dreams of the small, content life he shared with Raki and Priscilla.

Notable abilities make Isley one of the most formidable beings in his world. As an Abyssal One, his yoki energy was said to be even greater than that of Riful, making him arguably the most powerful of the three original Abyssal Ones. His awakened form is a massive, sleek, black centaur approximately ten meters tall. Despite his enormous size, he possesses incredible speed that outclasses even the fast Rigardo. His greatest and most versatile power is the ability to shape-shift his arms into any weapon he desires. He has been seen forming them into a lance, a battle-axe, a bow capable of firing homing arrows, a shield, a massive claw, and finally, a colossal claymore sword, which he considered his weapon of last resort. He also possessed immense regenerative abilities, even as an offensive-type warrior, allowing him to recover from grievous wounds, though even this had its limits against the relentless Abyss Feeders. Even in his human form, his attacks are powerful enough to destroy large sections of a mountain, and he retains the superhuman speed and strength that mark him as an unparalleled warrior.