Mark Underwood (formerly known as Harmony Williams) plays the acoustic guitar and provides vocals for Yeti. John Hassall met Underwood through his ex-band mate and Underwood's ex-flat mate, Carl Barat. Underwood is known to have taught Barat how to tap dance while they shared quarters. When asked in an interview about how he met Hassall, Underwood replied, "When John was in The Libertines, and it was going a bit AWOL, he wanted guitar lessons to make him more proficient as a lead guitarist. I was teaching guitar at the time and he happened to get recommended to me by a friend of his."
Mark Underwood has been quoted as saying that his influences in Yeti include the American alternative rock band They Might Be Giants, and when told Yeti sound like "the best part of The Kooks," he replied, "Do The Kooks have a best part? What is that, the bit before they play or the bit after they stop playing? Or the bit where they die in some propeller-in-face style accident?”