In the history of the X-Men, there might not be a more forgettable mutant than ForgetMeNot. Not to be confused with Forget-Me-Not from the pages of Runaways, this former X-Man is a mutant with the power of imperceptibility, which means when he is not in the direct view of a person, they immediately forget he exists.
This mutant's first appearance came in the Simon Spurrier, Mike Carey, and Christos Gage penned X-Men Legacy #300 with a story titled "ForgetMeNot." The mysterious mutant no one can remember seeing shows up and announces he is an X-Man. He explains his powers to an intruder who is caught in the X-Men's defense system and admits it is hard to make her notice him at all. When he sees the girl is stuck and can't get loose, he tries to distract her from the danger by telling her his story.
ForgetMeNot admits that he has been a member of the X-Men for six years at that point. He realizes the girl's right side of her face is ravaged by a vial of acid thrown at her by a boy she rejected. The girl said she wants to disappear for good. To this, ForgetMeNot admitted that he just wanted to be noticed throughout his life.
At this point, ForgetMeNot describes more of his powers. Not only is he completely forgettable to anyone who looks away from him, but his power also affects people's visual perception, aim, and weapons telemetry. This means anyone shooting at him or in his direction will miss. Even when he goes to the team leaders, like Magneto, and warns them something bad is coming, they don't remember the discussion at all when he leaves. He also points out that many Dues Ex Machina events in X-Men comics, where things conveniently happened were because he was there and was actually responsible.
This also led to ForgetMeNot having to explain how he was even a member of the X-Men if no one remembered him. Professor Charles Xavier brought him onto the team and then set a psychic alarm to go off once an hour to remind him of ForgetMeNot's existence. When Professor X died, ForgetMeNot took it harder than most people because the only person who remembered him was gone.
However, it didn't take long for ForgetMeNot to realize that his curse was also a blessing in some ways. When he found Mimic and Omega and asked them to help take away his powers, the powerful mutants told him that his ability to help others was a blessing not to be taken lightly, even if no one ever remembered what he had done. It was their influence that helped ForgetMeNot choose to retain his powers and help whoever he could.
His only other appearance was in Simon Spurrier and Tan Eng Huat's X-Force #10 when he saves the team from the traitorous Fantomex. Once again, his powers helped him save the day, even though his teammates will probably never know.