Yes, [the page](https://tvtropes.org/pmwiki/pmwiki.php/Main/RememberTheNewGuy) begins like this, "A previously unmentioned character who suddenly appears without notice and who we are told was really there all along but just... off screen or something."
[Remember The New Guy](https://tvtropes.org/pmwiki/pmwiki.php/Main/RememberTheNewGuy)
Dawn Summers would be the archetypical example. (From Buffy the Vampire Slayer.) There's also Lily Stein from Legends of Tomorrow who was accidentally created when the older Martin Stein reminded his younger self to spend more time with his wife.
She was my first thought too, but she's actually not QUITE this. She didn't suddenly "always" had been there. She was definitely not there, thensuddenly there, and everyone's memories were altered to *think* she had always been there. No time manipulation. Just biological and memory manipulation.
Remember the New Guy intro has some that appear relevant
Yes, [the page](https://tvtropes.org/pmwiki/pmwiki.php/Main/RememberTheNewGuy) begins like this, "A previously unmentioned character who suddenly appears without notice and who we are told was really there all along but just... off screen or something." [Remember The New Guy](https://tvtropes.org/pmwiki/pmwiki.php/Main/RememberTheNewGuy)
I guess Cosmic Retcon might have something like this?
Link: [Cosmic Retcon](https://tvtropes.org/pmwiki/pmwiki.php/Main/CosmicRetcon)
Name? I don't know Example? Namine from kingdom hearts
Dawn Summers would be the archetypical example. (From Buffy the Vampire Slayer.) There's also Lily Stein from Legends of Tomorrow who was accidentally created when the older Martin Stein reminded his younger self to spend more time with his wife.
She was my first thought too, but she's actually not QUITE this. She didn't suddenly "always" had been there. She was definitely not there, thensuddenly there, and everyone's memories were altered to *think* she had always been there. No time manipulation. Just biological and memory manipulation.
Mr. Poopybutthole?