I'm looking for a character off of very little description. My friend said her friend cosplays this character that holds a large flag, with the flag supposedly having a flipped swatstika or similar image on it? She says they're from a niche anime, and the only vague idea I have on the time period the anime could be from is that the person is in their 30s and they're very obsessed with the character so I have to imagine it could be an older show?

Sounds like Tokyo Revengers. The symbol you're thinking of is called a manji.