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.