Inquisitor Guillaime is frequently encountered by the player, Alphinaud, and Cid Garlond as they search for the airship Enterprise in Coerthas. In his first encounter, he brusquely warns them to stay out of Ishgardian affairs. Soon after, Lord Francel de Haillenarte is accused of heresy due to the presence of a Dravanian rosary being discovered in his belongings. Guillaime prepares to execute him at Witchdrop, but is stopped due to intervention by the player and Lord Haurchefant Greystone, who reveals Francel and House Haillenarte are being framed and the true culprit is seemingly revealed as a knight at the execution who summons a wyvern.
Later, while they try to get Lord Drillemont de Lasserrant's permission in searching the Stone Vigil for the Enterprise, Guillaime begins actively interfering with their attempts to gain support from the Whitefront garrison. Alphinaud ruminates over a flaw in the testimony from a guard about Guillaime's credibility, due to the specifics of the terrain around Whitebrim Front. Searching the nearby ravine, a corpse identical to Inquisitor Guillaime is found buried in snow.
Now with evidence that "Guillaime" is an impostor, they present this discovery to Lord Drillemont, who prepares to arrest the false inquisitor. In a confrontation at Snowcloak, "Guillaime" denies Lord Drillemont's accusations of heresy, on the grounds that he "cannot betray that which he owes no allegiance." It becomes clear that the impostor is actually one of the harriers who assumed the inquisitor's identity to take revenge on Ishgard. In the ensuing battle, the impostor summons draconic allies before transforming into a dragon himself as a consequence of his lineage. But the imposter eventually overcome by Drillemont and the Adventure, defiant to his last breath as predicted the Adventurer's death inside the Stone Vigil.
Guillaime is an Elezen man with black hair and slight facial hair under the lips.
Guillaime appears in The Palace of the Dead as an enemy known as "Insistent Inquisitor".