You have to play through 2K Showcase and MyCareer and fulfill certain objectives that are listed here.

Character	How to unlock
Adrian Neville	Get Promoted to RAW in MyCareer.
Alberto del Rio 2011	Successfully Complete All Objectives for Money in the Bank 2011 in 2K Showcase.
Batista 2003	Successfully Complete All Objectives for Armageddon 2003 in 2K Showcase.
Bill DeMott	Win Against Every NXT Superstar in MyCareer.
Bo Dallas	Get Promoted to Smackdown! in MyCareer.
Booker T. & Chris Jericho 2002	Successfully Complete All Objectives for the November 4, 2002 Episode of RAW in 2K Showcase.
CM Punk 2011-2012 & John Laurinaitis	Successfully Complete All Objectives for Money in the Bank 2011 in 2K Showcase.
Corey Graves	Get Promoted to Superstars in MyCareer.
Daniel Bryan 2012	Successfully Complete All Objectives for the August 13, 2012 Episode of RAW in 2K Showcase.
Kane 2002	Successfully Complete All Objectives for the October 28, 2002 Episode of RAW in 2K Showcase.
Kevin Nash & Ric Flair	Successfully Complete All Objectives for Backlash 2003 in 2K Showcase.
Randy Orton 2004	Successfully Complete All Objectives for the June 7, 2004 Episode of RAW in 2K Showcase.
Rusev and Lana	Get Promoted to NXT in MyCareer.
Sami Zayn	Get Promoted to Main Event in MyCareer.
Shawn Michaels	Successfully Complete All Objectives for SummerSlam 2002 in 2K Showcase.
The Rock	Successfully Complete All Objectives for Royal Rumble 2013 in 2K Showcase.
Vickie Guerrero	Play and Win a Match in the Smackdown Arena.
William Regal	Successfully Cause Him to Retire in MyCareer.