⚔️Serberia

It doesn't matter whether you write it into the character's backstory or after the game has already started. In this case, it's either one or the other. The one that isn't chosen in these situations can be learned later, but only if the opportunity arises.

Added 12 minutes later
You could also write that he worked at Cecilia's shop; that would be the most logical explanation for acquiring those skills, plus she's very mercenary, so she probably wouldn't just waste her time training someone for free.