Keyaru originated from Alban Village before being chosen as the Hero of Healing at age fourteen. Initially kind-hearted and aspiring to save the world, he grew hesitant upon discovering his healing power absorbed patients' pain and memories. This naivety led to exploitation by the Jioral Kingdom's royalty and fellow heroes, subjecting him to years of imprisonment, torture, sexual abuse, and forced drug dependency. Witnessing the destruction of his village and the deaths of his loved ones, including his foster parent Anna—raped by Knight Captain Leonard and driven to suicide—eroded his remaining humanity, leaving Keyaru as the sole survivor.

After developing resistance to drugs and pain, Keyaru escaped captivity. He later defeated the Demon King, revealed as the silver-haired Eve Reese, and claimed the Philosopher's Stone. Rather than surrender it, he used its power to reverse time four years, retaining all traumatic memories. Returning to the past as "Keyaruga" (or "Keyarga"), he embarked on a calculated campaign of revenge against his abusers.

Keyaru's personality shifted from gentle to vengeful, sadistic, and manipulative, though he maintained a rule against harming innocents. He expressed sympathy for others seeking justified revenge, often aiding them in exchange for assistance. His primary abilities stemmed from Healing magic, expanded into versatile techniques: Recovery Heal restored bodies to optimal states while enduring the target's pain; Imitation Heal copied combat skills and knowledge from healed targets; Looting Heal stole experience points or mana to level up or incapacitate foes; Corruption Heal inverted healing to internally destroy targets as a one-hit kill. Upgrade modified his or others' bodies for stat augmentation, shapeshifting (including gender alteration), memory manipulation, or object rewriting. Additional abilities included drug and pain resistance, precognition via the Jet Black Eye, and combat skills like the Clyret Style "Flower of Blood."

His revenge methods were extreme: First Princess Flare Arlgrande Jioral was tortured, raped, memory-altered, and transformed into "Freia," a devoted slave believing herself his lover. Knight Captain Leonard was transformed into a woman and gang-raped to death by brainwashed soldiers. Blade, the Sword Hero, was fed to aphrodisiac-drugged assailants. Second Princess Norn Clatalissa Jioral was raped, memory-wiped, and remade into "Ellen," a sister-like figure. Bullet, the Cannon Hero, was hunted for his role in Keyaru's rape.

After overthrowing King Prome Jioral, Keyaru became monarch intending to reform the kingdom into a peaceful realm. He maintained a harem including Freia, the ice wolf demi-human Setsuna, demon candidate Eve Reese, Ellen, and swordswoman Kureha Clyret. Though internally referring to them as "toys," he displayed protectiveness when they were threatened and occasional moments of diminished vengeance during their travels, suggesting unconscious emotional attachment. His warped morality persisted, exemplified by manipulating allies via aphrodisiacs or exploiting their trauma.