Newly introduced characters that have actually been around for a long time in-universe, but nobody remembers them usually because people's memories have been erased (Also all of my examples are from Marvel.)

**The Sentry** (Marvel Comics) Robert Reynolds used the be the most powerful superhero on the planet and a prominent member of the superhero community until all of humanity was forced to forget about him to prevent his evil dark side, the void from killing everyone.

**Spider-Boy** (Marvel Comics) Bailey Briggs was a kid who was experimented on by an evil scientist and given spider powers and then mentored by Spider-Man. He was erased from existence by a magic dagger created by an interdimensional wasp demon and brought back to a world where nobody, even Peter or his own parents, remembered he existed.

**BONUS REVERSE TROPE**

Newly introduced characters who haven't been around for a long time in-universe, but manipulate the memories of others to make them think they've been there the whole time.

**Voyager** (Marvel Comics) Va Nee Gast, the daughter of the Grandmaster(powerful cosmic entity in Marvel comics) was sent to earth for a contest involving the avengers. She manipulated their memories to make them think she was a founding member of the team.

**Xemnu** (Marvel Comics) Xemnu is a psychic alien criminal who can embed himself in the memories in humans, making people remember him as a beloved tv star and superhero in the place of the hulk. He used his powers to prey on people's nostalgia and fears.



I've been here the entire time, don't you remember?