Here is a website with all of the hidden characters and how to unlock them - http://master_phred.tripod.com/punchout/nessecr.html
Doc Louis *
I just recently discovered that there was a hidden training mode in the game. You need a game genie and just enter the code APLQSVTA and go to a NEW game. You will instantly start off with Doc Louis in the ring. He tells you to dodge a punch and counter punch. You do that and then he just stands there and you can beat the crap out of him. He has infinite energy, so you will have to reset the game. But it is cool to actually see Doc in the ring.
Mr. Scribbles *
This is actually more than a glitch rather than an actual character. To get this to work, you keep pressing the RESET button over and over. Then watch and see Mike Tyson’s face (or the first match with the newer punch-out). If Mike Tyson’s face is all glitchy, then go to the password screen and enter the code to get to the second Piston Honda. Piston Honda will look like a scribbled mess and every other character will be glitchy or invisible. The game can freeze a lot with this trick.
The best fighter on the game is also the hardest to find in the game also. To get to F.A.S, you have to beat the Another World Circuit 1500 times. ....... Try it sometime.
SW7 Destroyer *
To get to the destructive robot, you must enter the code 009 254 646 and before you enter the last digit, hold the controller in your right hand and start physically doing the robot in front of your television screen. Then enter the last digit which is a ‘6’. If it doesn’t work, open a few windows so .. you know... for good luck or something.
Strauss Corfam *
This is how to get to the secret boss of the game. The version must be Tyson’s Punch-out. Go to Mike Tyson. Obtain a score of over 8000. Then after Referee Mario makes his decision, press and hold A and B. Then let go when the Mr. Sandman picture appears. Then after that, hold RESET for 5 or more seconds. Go to the code screen and press 007 373 596 and leave the last digit 3. Then press RIGHT, SELECT, A and B. Right after, let go of the buttons and hold left button rapidly. When you hear a beep, hold the right button until the profile screen shows. If you did it right, you will fignt the Ultimate Strauss Corfam in the Death Match!!!