The challenger Yamanaka has had an interesting past. According to her wiki page, she was a dominatrix that got into MMA because "she thought it would help her in providing better service to her clients". Wow. Turns out she is pretty good at MMA, and has won 8 fights in a row. She's also avenged her only loss to Hitomi Akano by defeating her in their rematch.
Santos hasn't competed since June 2010 when she defeated Jan Finney via TKO in the second round. She originally won the Strikeforce women's 145 lb. belt back in August 2009 when she defeated Gina Carano in one of Strikeforce's highest-rated bouts ever (at least on Showtime). Carano is also rumored to be competing on the December 17th card, which will take place in San Diego, CA. The event will be main-evented by a Strikeforce lightweight title fight between Gilbert Melendez and Jorge Masvidal.
More SBN coverage of Strikeforce: Melendez vs. Masvidal