The key to unlocking hidden secrets in Final Fight One is in defeating Mad Gear members. As you play through the game, the Mad Gears you defeat will be added to a total, and when a certain number is reached, a secret will be unlocked. Here is a list of what you can get and the number for it:

50 opponents defeated: Alpha Guy unlocked

500 opponents defeated: Alpha Cody unlocked, lives select unlocked

800 opponents defeated: Level select unlocked

1300 opponents defeated: Color select unlocked

2000 opponents defeated: Rapid punch unlocked