Would it be really stupid to use Harry James Potter-Evans-Verres as the fictional character that had an impact on me for my CommonApp essay? On one hand it seems right since he introduced me to lesswrong which has certainly had a big effect but on the other hand... it's... you know... fanfiction.
You can do it. It's good countersignaling. But you have to be absurdly careful about writing quality. It's your job to convey to a skeptical audience that fanfiction can be transformative. You have to be absolutely brutal in avoiding language that signals immaturity -- or, better, find an editor who can be absolutely brutal to you.
My M.O., back in my college-essay days, was to read a New Yorker before sitting down to write. Inhale the style. Better yet, find some essays by Gene Weingarten, the modern master of long-form narrative journalism. Imagine what Gene Weingarten could do with HP:MOR. Then try to do it.
If it's worth saying, but not worth its own post (even in Discussion), then it goes here.
If continuing the discussion becomes impractical, that means you win at open threads; a celebratory top-level post on the topic is traditional.