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Altering established story facts is precisely what a retcon is.
Yes, they did just stick a new one in, using a trope that has already been ridden to death: memory loss.

Maple Story's story has long reeked of patchwork and lack of forward thinking. To doubt that this Hero was last-ditch effort to generate interest is to give credit to a team that had to delay a reveal countdown.

Technically, whoever erased Eunwol from history performed the retcon, as they altered the historical facts for the sake of continuity. Who cares if their introduction method is a trope, I'm happy they included a pirate Hero. Focus your negativity towards where it is deserved: Nexon's marketing and sales team.