- Not to be confused with "Sins" (The Seven Deadly Sins)
Sin「シン」 is a mysterious talking fox.
Sin appears a normal fox with a coat of pink fur covering most of his skin except for his underside and the tip of his tail. He wears a collar with a small, crystal pendant around his neck and his eyes always appear closed.
Sin was shown to be blunt and straightforward in his conversation with Percival and Donny, after saving them from Pellegarde. At first, he showed no interest in Percival as he didn't bother speaking with him until he had reason to believe that the boy was one of the Four Knights of the Apocalypse, as he claims he is part of a group who opposes King Arthur and the Holy Knights of Camelot who hunt down for the four in hopes of preventing the prophecy of the world's destruction at their hands.
His attitude shows he approaches matter in a business-like manner and casually dismisses Percival's retort that he won't destroy the world, claiming that despite what the boy intends, it is destined to happen and cannot be prevented.
When Percival and Donny were about to be killed by Pellegarde, Sin arrives with Percival's things and uses an Incantation Orb to teleport them to Dragon Back-Bone, a mountain 30 miles from the town they were in. Sin then lets them know that he saved them, leaving Donny shocked to see a talking fox. Sin then reveals that Percival is one of the Four Knights of the Apocalypse, four Holy Knights prophesied to destroy the world, and that he was sent to find them. Percival denies being such a thing and that he only seeks to find his father, to which Sin tells him that he is in Camelot. Donny explains that Camelot is a kingdom that was destroyed during the war against demons 16 years ago, but Sin states that Camelot still exists but that to get there they must first go to Liones. Donny doesn't believe him, but Percival decides to go with Sin and convinces Donny to join them.
On the way to Liones, the trio decide to stop for a meal, which Percival suggests a competition to see who catches the biggest prey. Donny fails to capture any prey with the bow, earning Sin's taunts. In that, a scream after a lost arrow leads them to meet a young giant woman that surprises Sin for being a giant who worships Goddesses but not nature. When some fairies appear talking about a boy who got into the Echo Gorge, Donny tells them that it must be Percival. Fairies explain that Echo Gorge used to be beautiful but now it's hell.
After meeting with Percival and defeating the Holy Knight Talisker, Sin explains that he used a Chaos Staff that the king of Camelot gives to his knights. Sin reveals that he captured a prey that gives him the victory in their competition. After Nasiens joins the group, Sin says they need transportation to get to Liones, so Nasiens suggests going to Sistana to get it,
When the group stops to eat, they discussing the events that Percival, Donny and Sin went through before meeting Nasiens. Donny berates Sin for not having used his only Incantation Orb to take them to Liones.
Upon reaching Sistana, a mini golem leads them to find a piece of the Coffin of Eternal Darkness buried., surprising Sin for having searching properties.
At night, the four of them stay at an inn. Sin tries to explain what the Coffin of Eternal Darkness is, but is unable to finish because of Donny's interruption. They then spy on Ironside's speech to the townspeople, revealing that he owns the other pieces of the Coffin. Sin reveals to them that Ironside plans to kill everyone in town to activate the Coffin's magic. Sin suggests that they escape with the missing piece, but Percival sets out to save Anghalhad when she interrupts the speech. Nasiens, Sin, and Donny run to the inn, but discover the maiden stealing the piece, actually being Ironside's familiar in disguise.
After the battle with Ironside, Sin arrives stating that his allies got rid of the monsters that Ironside had summoned throughout the town. When Donny and Nasiens ask him about it, Sin says they are his "forest friends", something only Percival believes to be true.