Mercio is one of the Six Fragments of Chaos. He is a lycanthrope, a non-human creature that resembles a sort of wolf, also known as a werewolf. Unlike present lycanthropes, Mercio stands on two legs, wears clothes, and is able to talk and fight. This may come from his existence in Ground Zero, where other lycanthropes had the same ability, or more likely because he is part of chaos.
As one of the Six Fragments, Mercio is immensely powerful, able to fight on equal footing with Id. He also has a high degree of durability, as he is able to withstand many high level attacks from Id and Lamia. Id also notes that Mercio cannot die, probably due to the events that took place at Ground Zero.
10,000 years in the past, Mercio and the other Six Fragments of Chaos lived in Babel, one of the last remaining citadels of Ground Zero. He is first seen when Ashirijen and Chaos battle one of God's angels (turned demonic) along with Kukudo, both of whom were thought to be dead. The four team up to battle the minion, which quickly loses interest in them and refocuses its attention on Id, who had defeated one of its companions. Mercio shows considerable surprise when Id defeats it as well.
Later, when the Six discuss their plan of attack, Mercio suggests using the "weak," remaining citizens of Ground Zero as shields. He explains his reasoning by pointing out that the citizens want to fight despite not having powers, and says that it might be more prudent to use them as assets to battle God. Ashirijen reprimands him, and Kukudo cuts in to change the subject. Upon Id's return, Mercio calls him "one of God's dogs" and a "chick", to which Id calls him "Yellowy", which irritates Mercio. He questions Id's plans, but is interrupted when an Ancient Dragon and God's minions launch a surprise attack on Babel. After Percer defeats the Ancient Nova and Ashirijen clears a path, he along with Id, Kukudo, and Chaos, run ahead to go and face God.
Mercio and Kukudo end up facing off with one of God's higher minions, upon being declined permission to visit God. While Id and Chaos rush ahead, Mercio and Kukudo engage in battle with the minion. They fight with the minion and bring it to the ground. Just when Kukudo goes to deliver the finishing blow, the minion blasts him away, causing him to disappear and presumably be sealed inside the Red Stone. Angered, Mercio tries to finish what Kukudo started, only to also be defeated, his last words being: "for me, this was too much." He then turns into light and disappears.
Mercio is shown to reappear with Chaos before Id's battle with Dark Mage Cleon. He remembers little of what happened before, mentioning that he feels "like [he] doesn't belong in [Gressen]." The two talk about how boring life is, serving as Geruman's underlings, and their lack of memories. The conversation is cut short when they feel the (Red Stone) Seal break, and Chaos leaves to go see what happened, leaving a trace of evil ki behind.
He later appears at the palace in Ballapar, capital of Kanon, which the Six Fragments of Chaos are staying in. When Chaos and Kukudo leave to go and face Id (and subsequently Selenia and Irlina), he comments that he isn't some sort of guard dog, but stays regardless.
When Chaos returns, Mercio reveals that Duke Bahajan had escaped from his grasp. He doesn't treat the matter seriously, beliving that Bahajan is hardly a threat, which angers Chaos to the point that he attacks Mercio. He threatens him to find Bahajan and capture or kill him, else his own life is forfeit. Whilst pursuing Bahajan, Mercio encounters Id's group along with the former. He engages in battle with two Great Silvers, Bahajan himself and Bellephor. Both men are shocked at how fast and powerful he is, and Id notes that Mercio has become even more powerful than his Ground Zero self. He is easily able to defeat them, but was stopped from any killing blows by Id.
He doesn't remember Id, but is surprised at how powerful he is, and still believes Id to be a girl. They fight a high-level battle; Mercio is momentarily caught off-guard when Id's group calls out to him, as this is the first time he hears Id's name. Id takes advantage of this and attacks him relentlessly. When he stops, Mercio says that they are allies, working together with Chaos, and that as a group they can achieve the goal they fell short of in Ground Zero. When Id replies coldly that he has no intention of joining them, Mercio gets angered and says he'll have to drag him by force. They continue fighting. In the middle of the fight, both Mercio and Id stop for a moment, allowing Mercio to stretch. He says that he hasn't fought for 10,000 years and is feeling stiff, and it would be a shame if he couldn't fight properly, to which Id agrees.
Mercio asks Id one more question; despite having different ideals, they both fought against God, which means they shared a common goal. He asks why Id won't join them and sort out ideals later, to which Id replies that he can never forgive Chaos for sealing away the citizens of Ground Zero. Mercio says this is a lie and that Chaos fought until the end for the citizens, and that he refuses to believe a tactic that would turn him against his leader, and the two continue fighting. The fight takes an even more serious turn, and on almost being defeated by Id, Mercio decides to try and fulfill his mission instead of battling further. He attempts to kill Bahajan by creating an explosion and satisfied, he leaves.
When he returns to the castle he runs into Ashirijen and Chaos. Ashirijen shows concern and shock over the serious wounds that Mercio had sustained from his battle, but Mercio brushes her off, focusing on Chaos instead. He asks Chaos what really happened at Ground Zero; when Ashirijen asks what the meaning of his question was, Chaos knocks her unconscious. Mercio quickly catches Ashirijen and shouts at Chaos, and upon learning that Chaos sealed away the citizens for his own personal gain, uses Silver Cuspid to attack him. Chaos retaliates and brutally attacks Mercio, who had put Ashirijen to the side for her own protection. When she wakes up later, she finds Mercio trembling, terrified, in front of Chaos.
Later he is shown to be standing in the center of the palace at Ballapar, seemingly unharmed.
- Silver Cuspid - Mercio's primary attack; he manipulates ki shaped in a tail-like formation to strike and pierce through others. Mercio notes that it isn't a corporeal attack, making it difficult to defend against.
- Wild Spirit - an attack that releases flames, that shape themselves into a head similar to Mercio's own. It is an offensive attack. It creates powerful shockwaves as an after effect.
- Stamp - creates 'stamps' in the air and objects that explode when touched. From Id's comments, it can be inferred that it is as strong as a dragon's breath attack.
- Scarlet Field Burst - a giant burst attack that can be used as both a shield and an offensive attack.
- Guillotine Claw - a long range attack that creates claws, presumably out of ki.
- Wind Cutter - an air attack that creates the affect of slicing through things.