Actually (spoilers) it was her real name! And she only told Emmet.
Anglerfish162 wrote: in the essential guide
I have the essential guide, but I couldn't find the word "Lucy" in her profile page. It did describe her past aliases, but didn't say her birth name. In fact, there was a picture of Emmet asking "Is that your real name" and Wyldstyle replied "Let's not talk about my name." So, the essential guide hid that important detail.
Then why say "essential guide"? I guess it could be a slip of the tongue, but then again "Lucy" could have been mentioned somewhere other than her profile page. We might as well just let Anglerfish explain instead of speculating what he was talking about.
Oh, it's her section in the "Build the Better World page". Maybe I should have checked that before.