Sarah Urist Green, Wife of nerdfighter co-founder John Green. She is called the Yeti because "she is so hairy"-brotherhood 2.0 video August 30th.
She is also in the dedication of an Abundance of Katherines. (Maybe other books, I don't know) as well as the Acknowledgements. The Yeti is sometimes seen in glimpses in some Brotherhood 2.0 videos or the post-B20 videos. She is also heard in many videos.
"I'm John" says Hank
"I'm Hank," says John
"and I'm the Yeti" says the Yeti