Using the DISGUISE command, you can change your appearance to look like a normal civilian of the same gender, thus fooling most evil-doers.
Of course, you can't change into your civilian disguise if there are enemies nearby enough to see you unless you use the traditional place where superheroes are supposed to go when they need a discreet place to change.
While in disguise, you won't have any power at all and you'll be limited to normal civilian land speed. Well, flying or shooting fire from your eyes wouldn't look totally unconspicious, would it? To revert to your superheroic form, use the TAKE OFF DISGUISE command.
Any damage will automatically revert you to your true form (and transfer the damage).
While this attribute can't be customized using the FFX Control Center, custom entries can be added in ffx.py under FFQ_DISGUISE_TEMPLATES. You'll also need to add the corresponding entries in FFEdit; make certain you copy the current entries settings.