Tekken Advance

Change costumes
Select your fighter, then press the A, R, or Start buttons. Each button will provide a different colored costume for your fighter.
(Thanks to Guiltyofgaming@aol.com)

Unlock All Characters
Press and hold A and B. Then press L, R, R, L, L, UP, UP, R.
(Thanks to Guiltyofgaming@aol.com)

Unlock Heihachi
Beat the game with all 9 characters.
(Thanks to IInsulator@aol.com)