There are 14 unlockable characters in Tekken Tag Tournament (18 including Panda and Monkujin, Angel and Alex). To a characters you must play arcade mode and pass all 8 stages. Once you have done that you will unlock a character. You must beat the arcade mode 14 times to unlock all characters.
Unlockable characters: Kunimitsu, Kazuya, Bruce, Kuma/Panda, Jack-2, Lee, Tetsujin/Monkujin, Wang, P. Jack, Devil, True Ogre, Ogre, Roger/Alex, and Unknown
*NOTE: To select Tetsujin/Monkujin in arcade mode you Must hover over Yoshimitsu and go down or hover over Wang and go right (same principle apply to Unknown. You must hover over Lei and go down or hover over Roger/Alex and go left)*
There are also hidden characters. These characters are Panda, Tiger, Monkujin, Angel, and Alex.
To select these characters you must hover over certain characters and push a certain button
*NOTE: The characters have the same exact moves and fighting styles, This is basically an alt costume*