How should I refer to a character who's name hasn't been revealed to the characters yet?

Basically to bring people up to speed, I'm writing a Honkai Star rail fanfic, that has the character Firefly in it, but it takes place before the events of the game when she went by another name [which is SAM who is kinda like her alter ego in a sense] for the general reader who might not know this information should I call her by the name most people would know her as.

My main issue is I want her getting said name to be a bigger moment and don't want to ruin that by calling the character Firefly throughout while the other characters within the story all refer to her as SAM I feel like it would go against all that build up I want to create

Sorry if this sounds really rough and convoluted, I find it hard to but my questions into writing, if you need more details please just ask because I want to make sure I'm doing this right and not shooting myself in the foot