To enter a hidden retro level, use a taser weapon on the indicated arcade machine in the listed stage. The taser can be obtained from cops, or found on the ground, nearby the arcade machine. If you do not see a taser, smash all the crates and other item containers nearby until you find one.
Stage 2: In the prison, past the guards with tasers, is an open doorway in the background with the arcade machine.
Stage 4: Look for an entrance door on the docks, and go left to find the arcade machine in a small room.
Stage 5: The arcade machine is in the bar, after exiting the sewers.
Stage 8: After leaving the Art Exhibit, you will enter a back room with a taser on the ground. Use it on the nearby arcade machine.
Successfully complete the indicated task to unlock the corresponding character:
- Adam Hunter: Successfully complete Chapter 4.
- SoR1 Axel Stone: Reach 200,000 lifetime score.
- SoR1 Adam Hunter: Reach 250,000 lifetime score.
- SoR1 Blaze Fielding: Reach 310,000 lifetime score.
- SoR2 Max Thunder: Reach 390,000 lifetime score.
- SoR2 Axel Stone: Reach 480,000 lifetime score.
- SoR2 Blaze Fielding: Reach 570,000 lifetime score.
- SoR2 Skate Hunter: Reach 650,000 lifetime score.
- SoR3 Axel Stone: Reach 730,000 lifetime score.
- SoR3 Blaze Fielding: Reach 840,000 lifetime score.
- SoR3 Skate Hunter: Reach 940,000 lifetime score.
- SoR3 Dr. Gilbert Zan: Reach 1,050,000 lifetime score.
- SoR3 Shiva: Reach 1,150,000 lifetime score.