I'm looking for a title based on one characteristic.
The protagonist was created by the antagonist.

Examples:
Protagonist is an experiment created by the antagonist
Protagonist is a clone of the antagonist
Protagonist is an antagonist who has a split personality
Protagonist is the antagonist who has lost his memory
Protagonist and antagonist were once one person, but have split into two
Protagonist lost his memory because of the antagonist
Protagonist is the reincarnation of the antagonist
Protagonist and antagonist switched bodies because of the antagonist
Protagonist is an antagonist from another time

Etc. Any options, except the "relatives" option.

I would be grateful for an answer.

What about time rewind revenge manga? Something like Kill the Hero, Again My Life or The Return of the 8th Class Mage...?

~4400 manga completed
~1700 ongoing...

Lasboss x Hero has the author of a manga finding himself into his own story, where he was supposed to kill off the main heroine he himself created. It's mostly played for laughs.

Pandora Hearts kinda has shades of that, but that comes up very far into the story.

My current top 3:
Fullmetal Alchemist
Pandora Hearts
3-gatsu no Lion

Is an anime suggestion OK? I would suggest Noein