It depends on where you live, but knowing Japanese (especially hiragana and katakana, which you learn in high school Japanese class) is not terribly uncommon, and even quite common in some places (for example I'm right next to a town that's known around here to have a rather large Japanese immigrant population).
Why not just come up with some secret symbol representing her somehow? When people ask you what it is or what it means, you can just say "that's a secret". And it won't make you look like a dork to people who know how to read katakana.