Defensive ability is one of the properties of a character. Having a low amount of it can be likened to fairies, or paper.
The term refers to one’s tolerance to attack (hence it can be called “stamina”, although the word more often refers to life or dizzy capacity in fighting games), but the actual value of the defensive ability in a game’s programming comes in two different types (likewise, it can be called “defense” like an RPG stat when it clearly refers to the value and not the action of defending).
Whichever type it is really only concerns the developer most of the time; the player will only truly be able to judge changes in it when doing things like trying to judge how much chip damage is needed to get a knockout (however, training modes and such that display the amount of damage numerically will make it possible to judge).
Games with both kinds of defensive values are not rare at all; often, this is done to allow for mechanisms like desperation.
The general idea of a target damage rate is often introduced taking “100%” as a basis.
In this case, a lower numerical value would mean a higher defensive ability (in almost all cases, the underlying programming is only known to the developers, so this is all that can really be deduced externally). With this, when the same attack hits, there will be the visible differences in damage.
For the most part, basic values for defense tend to be taken from protagonist characters. Looks often make it easy to judge whether the defense will be higher or lower: those with big or stocky constitutions will usually be able to shoulder more damage at a time; smaller characters (and most women) will take more damage from the same hits. Also, characters who have particularly good attacks and speed will often have less defensive ability (popular examples are Gōki, Andy Bogard, Chip Zanuff, and Majikina Mina).
The alternate approach is that the whole value of life capacity varies; therefore, even though the same attack hits, it appears that there is a difference in damage value, so there also appears to be a difference in the defensive value.
For example, on a lifebar with the same number of pixels, a character with 100 hit points and a character with 120 hit points, when either takes 20 points of damage, will have one-fifth and one-sixth of their life taken away, respectively; thus, the number of pixels that go away will be different as well.