The PPC tries to keep themselves secret. Most of the time, the canons are ignorant. But some canons know.
1. Omnipotent/omniscient deities who can be expected to have knowledge of the full multiverse know without having to find out. Aslan, Illuvatar, and Madoka have been established to know of the PPC. Omnipotent/omniscient deities tend to be hands-off and to care about the free will, but will occasionally step in when they have been maligned directly.
2. Very powerful characters, though not omniscient, are more likely to find out. Death from Discworld is aware of the PPC; so is Father Christmas from Narnia and Ponch from Young Wizards. They tend to be the type not to interfere with agents' work, and to be quite cordial when encountered.
3. Characters who are insane enough to see through SEP fields and perceive the multiverse. Discworld's Altogether Andrews has one or more personalities that know about the PPC. The A-Team's Murdock has managed to retain vague memories between neuralyzations, probably because of his unusually flexible mind, but seems content to stay within his own world. Characters whose mental illness or insanity or strange outlook lets them see the PPC tend to also be pretty accepting of the PPC's presence, since it's no more odd than any other part of their lives, and are unlikely to be believed by other canons in any case.
4. Characters who are very, very good at analysis, to the point that they can detect tiny clues that the agents left behind before the canon snaps back. So far the only character in this category is Sherlock Holmes, who has on a few occasions figured out that something odd is going on. The more in-character he is, the more likely he is to use his detective abilities to suss out the PPC. Luckily, the closer to IC he is, the more likely he is to be fighting the Suefluence and to recognize the PPC as allies. He is usually neuralyzed at the end of a mission, but because of his abilities, is likely to figure it out all over again.
5. Characters who live in a metafictional continuum and can recognize the PPC as multiverse travelers. In this category: The characters in Gorillaz and the Doctor's TARDIS. Characters like Deadpool and the inhabitants of the Thursday Next continuum are likely to figure it out, too, but this hasn't yet been observed.
6. Gaspode and other minor characters who are both very perceptive and very easily overlooked. When they repeatedly see the PPC fix problems in their continuua, and the agents forget to wipe their memories because they're so forgettable, they may become allies. Gaspode from Discworld was the first, and can be expected to help agents. Luckily his awareness doesn't harm his home continuum because he canonically knows a whole lot more than he tells.
7. 1984's Big Brother, who was directly clued in by a rogue PPC agent. Unfortunately, Big Brother could be anything from a figurehead to a concept to an icon of the government of Oceania--he may not be a person at all. Luckily, 1984's technology is stuck at the 1950s Earth level thanks to constant warfare and a repressive government, and he's unlikely to ever make his way out of his own world. That does, unfortunately, mean that any agents doing a mission in 1984 have to stay constantly disguised and in character, making sure to use their technology only when they are not on camera. Whether Big Brother wants to steal PPC technology or actually invade the multiverse is unknown, but the PPC isn't keen to find out. Luckily 1984 badfics are rare.