According to this, both Amanda Warner and Brianna Wu know "Natalie", so if Natalie doesn't exist, they're both liars.
https://unepetitepeu.wordpress.com/tag/giant-spacekat/
http://www.theguardian.com/profile/brianna-wu
https://unepetitepeu.wordpress.com/tag/giant-spacekat/
The "Gaurdian"? Looks like she's been spending time around the Guardian offices after all...
http://www.theguardian.com/profile/brianna-wu