The "Grey Zone" is for morally-ambiguous individuals and organizations. They can be Anti-Heroes, Anti-Villains or both, and often act(ed) as neutral characters. Those characters often appear in mature and dark-oriented stories in which the traditional "good vs. evil" scenario does not occur.

Even if they're heroes, their morality is enough decayed to be questioned; their exact nature is debatable, and therefore impossible to truly determine, and it is up for the readers or viewers to make their judgment: while some can see them as heroes, even if they do not approve their methods, some will see them as criminal or even villains. One of the best instance is V who can be see either as a rebellion hero or anarchist, or Scorpion from Mortal Kombat who can be see either as a heroic or villainous Assassin.

The most unique case about grey zone characters is that they aren't beholden to good nor evil, and they can also be responsible for keeping both moralities in balance, namely both good and evil. On the other side of the coin however, is that they can passify good, and seek to redeem characters as much as benevolent characters do, and they can also seek to put an end to ongoing conflicts and peacefully end them, or else something horrible will take place. This is exactly what arguably makes neutrality of three different alignments much more popular than benevolent characters.

On very rare cases, characters that are truly and purely neutral also do exist, and examples of that will be Death from Mythology, the Living Tribunal from Marvel Comics, Truth from Fullmetal Alchemist, Sibylla from Winx Club, Zen-oh from Dragon Ball Super, Balancer from Valkyrie Crusade, and Lord of Nightmares/L-sama from Slayers.

This category also includes "neutral" characters who do not attend to do good nor evil but can be sporadically "heroes" (as Saul Goodman, the Heavy from Team Fortress 2 ).

On & Off characters who commit at the same time heroic and villainous deeds, such as some incarnations of Deadpool, Rick Sanchez or Michael Afton, or false heroes known as "Heroes by Proxy" (such as John Constantine) also belong to this category.

Their main co-alignment is Chaotic Neutral, as they almost always follow their own codes and rules (even if they're not heroic). They're rarely Lawful Good, though rare exemples exists (as L Lawliet).

HIGHLY IMPORTANT: Heroes who are Pure Good can never be in this category for they remain completely incorruptible whereas it is unknown if Grey Zones are bad or good.

Also, note that Redeemed Villains are not in the Grey Zone, no matter how bad they were, if they turned Pure Good.