Diodora is a significant character in the manga Four Knights of the Apocalypse, introduced as the son of Ironside, a knight of Camelot and the primary antagonist in the series' first major story arc. Diodora's origin is steeped in tragedy, as he was born after his mother was fatally poisoned by the lingering miasma of the Demon King from the previous holy war. His mother died, and Diodora himself was born with a deadly, incurable disease. To save his life, his father Ironside took him to the magical kingdom of Camelot, where the ambient power of Chaos allows him to live healthily. This has forced Diodora to spend his entire life within Camelot's borders, as leaving would cause his illness to return and likely kill him.

Initially, Diodora is portrayed as a kind, cheerful, and somewhat naive boy, described by his father as being honest and gentle, much like his late mother. He loves his father deeply and enjoys listening to stories about his mother from his uncle, Mortlach. He practices fencing under Mortlach's tutelage and shows considerable talent and progress, though he asks his uncle to keep this a secret from Ironside so as not to worry him. Despite his cheerful demeanor, Diodora is intensely curious about the outside world of Britannia, which his father has told him is filled only with dangers. This enforced isolation and the lies told to protect him begin to fuel a quiet resentment and a strong desire to see the truth for himself.

As the story progresses, Diodora's sheltered innocence is corrupted when he acquires a powerful and dangerous magic. This ability, which he names Despair, allows him to automatically reflect any attack directed at him back onto the attacker, but with a critical and defining condition. The reflection only activates if the attacker harbors negative emotions such as malice, killing intent, anger, or hatred. If an opponent attacks him with a calm and pure heart, devoid of any desire to harm, the ability does not work, and Diodora will suffer the damage directly. He also gains access to another ability called Invocation, which he takes from one of his allies, Turret. This power allows him to summon and control vicious Beasts of Purgatory.

The acquisition of this power has a profound and darkening effect on Diodora's personality. He becomes arrogant, reveling in his seeming invincibility and showing no remorse after killing his own allies, Rothes and Turret, with his reflected power. Instead, he takes a dark pleasure in their despair and coins the name for his magic in that moment, marking his descent into a more villainous mindset. His character arc positions him as a direct thematic foil to the protagonist, Percival. Where Percival's power, Hope, grows from the positive emotions of his friends and his own pure intentions, Diodora's Despair relies on and feeds upon the negative emotions of his opponents. Their conflict is framed as a battle between hope and despair.

This central conflict culminates in a one-on-one duel between Diodora and Percival. Despite unleashing the Beasts of Purgatory and relying on his Despair ability, Diodora is completely defeated. Percival, having recently overcome his own rage and shed all malicious intent, attacks Diodora without any anger or desire to hurt him. Because of this, Diodora's Despair magic fails to activate, and every one of Percival's blows lands, leaving him utterly vulnerable and defeated. Following his loss, Percival offers Diodora a hand of friendship, hoping to help him. Diodora, consumed by his own despair and believing he has nothing left to live for, refuses the offer and insists that Percival kill him. However, Percival brings up Ironside, reminding Diodora that his father, whose memories have been altered by Chaos, has forgotten him. Diodora is left emotionally broken, crying with the creatures he summoned, but Percival's offer leaves a small opening for potential redemption and reconciliation with his father.