(press Select). Otacon will give you the Stealth Camo at the end of the game, making you invisible to everyone except bosses and cameras.
Here’s a well-rounded, helpful review for a Metal Gear Solid 1 trainer (assuming you’re reviewing a PC trainer for the original game, likely the 2000 PC port or an emulated version): metal gear solid 1 trainer full
Ensure the trainer version matches your game version (e.g., v1.4.0 vs v1.0.0). (press Select)