Trusty John is a German folktale also called "Faithful John." In the original tale, John swears an oath to be loyal to the son of a dying king no matter what. The young king weds a princess, but on the way home John overhears three prophecies that will lead to his king's death. Each prophecy also includes a way to dodge death as well as an ill fate for anyone who warns the king of the danger. The three fates for anyone who warns the king are to be turned to stone: first from the feet to the knees, then the knees to the neck, then the head. Faithful John suffers this fate, but is brought back to life by sacrificing the king's son (who is also returned to life)
The doorman at the Woodland building, John was almost universally popular. Amiable and good-natured, John enjoyed his job and was always pleased to see everybody. As such, it came as a complete shock to everyone when Kay unmasked John as a spy for the Adversary. Baffled as to how the 'most faithful Fable in history' could turn against them, John was interrogated by Prince Charming, Beauty and Beast; he explained to them that, long before signing up to the Fabletown compact, he had sworn an unbreakable vow of loyalty to his King, who he believed had been killed leading his forces against the Adversary. It turned out that this had not been the case, that his King, now working for the Adversary, had contacted John and ordered him to spy on his fellow Fables.
Bound by his vow, which overruled the later compact, John had no choice but to obey and spent the next four years passing information to the enemy before Kay, with newly regrown eyes, caught him. Despite being sorely tempted to forgive him, given the circumstances, Charming realized that treason, no matter what the situation, must be dealt with harshly. Hardening his heart, he gave John the option to jump down the Witching Well under his own power as a gesture of mercy, the alternative being that he would be executed on the spot and his corpse put down there anyway. John accepted the former offer. Few people explicitly know what happened to him, as revealing such a beloved Fable to be a traitor was deemed to be bad for the morale of Fabletown.
On his mission into The Witching Well, as depicted in the story-arc The Good Prince, Flycatcher recruited John as his squire, offering him the possibility of redemption by accompanying him. Following the completion of his quest, Flycatcher, now King Ambrose of Haven, announced that John would henceforth be known as Trusty John once more, and appointed him to be Lord High Chancellor of the kingdom, putting him in charge of the day-to-day running of the new realm.