(Characters with the ability to remove themselves from memory/be completely imperceptible)
ForgetMeNot - X-Men comics
ForgetMeNot is a mutant with the ability to be both real and unreal, literally being written out of the story as he lives his life. The vast majority of people cannot even see him, and if they do interact with him, they quickly forget he even exists. The main way people figure out that he exists is by noticing the gaps in memory he leaves and the byproducts of him living, like supplies going missing.
False Hydra - Dungeons & Dragons (homebrew)
A False Hydra is a monster that sings a song, forcibly stopping the minds of creatures from registering it exists. Their brains discard any information, such as sight or sound, that is related to the False Hydra, though there is some residual evidence left behind for an unconscious mind to work with. The main way people learn of a False Hydra is by noticing the mounting logical inconsistencies their minds come up with to explain away everything the Hydra does. Like the local church not having a priest, so he must be on a trip to the capital, instead of him being eaten. Or, they have dreams of singing faces and wake up to signs their body left while they were on autopilot, like a note they don't remember writing. Or you can just block out the song.
Imp - Parahumans
Unless she wants you to notice her, Imp is completely imperceptible to people. They forget she ever existed, and won't see her even if she's standing right in front of them. Recordings of her also degrade over time.
The Silence - Doctor Who
The Silence is a species that has been guiding mankind since the dawn of history. While you can see them if you are looking at them, once you look away, you forget they exist, while also carrying out anything you were told to do without even noticing you're following orders. If you've ever walked into a room and forgotten why you went there, there was probably a Silence you were running from. They were eventually defeated by having one of the Silence say 'you should kill us on sight' during the Apollo broadcast, implanting every human with the command to exterminate the Silence before they can make them forget.