The player meets a coherent prisoner on the balcony of the Fool's Idol's church who promises that he will cause no trouble. The magical seal on the floor he is conjuring is remarkably similar to the paralyzing seal the False Idol casts during the boss fight. He is a liar, as soon as the Fool's Idol is defeated he will laugh and the spell he has been casting will resurrect the boss. The Royal Mistress warns of this "I know the truth, the church Goddess may die, but never for long." Kill the Liar to prevent the Fool's Idol's resurrection.
As he is the most powerful of the mage prisoners he was likely a royal of very high status before he was imprisoned. Since both the Liar and the Former Royal's Wife know the truth it has led to a theory that the Liar was once her husband. Perhaps after the jailing of the Queen's family the wife took the Queen's side and the Liar sided with the Old Monk, resulting in a falling out. Another possibility is the Liar could be serving the Old Monk in order to bring comfort to his wife, the Singing Lady, and she is speaking metaphorically that she is no longer married as the Liar has twisted into a being that is no longer her husband.
It is unknown if after the Old Monk was banished the Queen took a new husband. If she did, another possibility is that the Liar is her soul-starved widower.