Kerith is not in Dance Central 2. She debuts in Dance Central 3 (2012).
Kerith unlocks after completing all routines on all difficulties for the main campaign:
Correction: DC3 has 40+ songs. Kerith unlocks after completing Story Mode (all 8 crews defeated) + Master Quests for each crew. Not simply "all songs on all difficulties."
Enjoy the 2011 roster. Best characters for style: Mo (smooth), Glitch (popping), Emilia (technical), Dare (aggressive). DLC crews add variety.