In sci-fi action games we usually find this feature.
A secondary health that protects the main HP points, all will be damaged first instead of the main HP points.
For an example, take Halo series or Quake series.
Can someone do it?
Make it in RGSS please.
Effectively the Armor value found in Half Life games and such...
For an RPG though, I'd suggest something like the mage ability from World of Warcraft, Power Word: Shield. It absorbs a predetermined amount of health before damage is dealt to the character's HP.