To get these secret characters, you must beat the game a certain number of times.

Alternate Anna Outfit:

Play as anna 25 times in arcade mode.

Tiger:

To play as the superfly Tiger, beat Arcade Mode 16 times. Now highlight Eddy and press Start.

Gon:

To play as the dinosaur Gon, beat him in Tekken Ball mode.

Alternate Gon Selection - Another way to play as the cute dinosaur is to play Survival Mode well enough so that it asks you for your initials after you lose. Put in the letters GON and the next time you check the roster Gon will be selectable.

Dr. Boskonovitch:

Finish the Tekken Force Mode 3 times to gain 3 keys. Finish it again until you reach the 5th stage and face the doctor. Beat him and he becomes a selectable character.

Panda:

Highlight Kuma at the Character Select screen and press Circle.

Tekken Ball and Theater Modes:

Beat all 10 main characters in Arcade Mode to extract Tekken Ball mode and Tekken Theater mode

Alternate introductions:

Complete the game with the ten base characters to view an alternate introduction featuring the characters in their two-player mode costumes. For another one, complete the game with all characters including the secret ones.

Alternate winning poses:

To see your character's secret winning pose, hold either Kick button during the replay of your win. To see the original pose hold either Punch button.

Alternate costumes:
Play as any character the indicated number of times. Then, highlight that fighter on the Character Select screen and press Start:

Xiaoyu: 50 times
Jin: 50 times
Anna: 25 times
Gun Jack: 10 times
Replay Mode:

Go to Practice, then go to the Freestyle Menu. Press R1+R2+L1+L2+Circle for Recorder Mode (if you did it right, you should see a message on the screen). This lets you save and replay moves in practice mode.

Mokujin the block of wood:

After choosing Mokujin in any mode simply press and hold Down on the D-pad until the fight starts. Mokujin will now sound like a block of wood when he is hit.