I stand by my statement. If the essay asked about "the fictional character that had the greatest impact on you" or something to that effect and that person is HJPEV, then that's what you should write about. Otherwise, you'd be lying, and apart from the general wrongness of lying, you're going to write better about something that's true.

As someone currently going through this process (I just wrote the same essay about Terry Pratchett's character Tiffany Aching), the impression I get is that it's very important to be unique: if your essay is the same as 200 others, it will be penalized as much as if it is poorly written. Using a rationalist fanfiction character, if you can write it well and have the guts to write it sincerely (but not too sincerely, or you'll signal naivete), is a good idea. If you don't want to deal with a fanfiction character, write about some other rationalist. Either way, don't mention lesswrong. And please don't write about Howard Roark. I enjoyed The Fountainhead, but it's worse signaling than fanfiction. You'll look like a shallow thinker who falls for propaganda, and most universities lean to the liberal end of the spectrum. Important note: I'm applying to highly selective colleges with student bodies that think of themselves as contrarian or meta-contrarian. If you aren't, this advice may not apply.