Here’s a thing I’ve been forgetting to mention:

Imp’s power reminds me of the Gray Men (also known as the Soulless) from the Wheel of Time series. They’re servants of the Dark One who are nearly unnoticeable, and are consequently used as assassins. They’re not invisible, but the gaze of most onlookers, even ones who are actively on guard, will just slide right over them. In many cases, witnesses will report seeing the victim fall but not seeing the murderer, even though it happened right in front of them, in plain sight.

Imp’s power is stronger, though, since even if she does get noticed, her power can then cause people to forget about that shortly after. Gray Men are hard to notice, but they don’t mess with your memories once you do.