Evil Counterpart/Tabletop Games

Examples of s in include:

Board Games

 * Warhammer 40,000: Space Marines and Chaos Marines, Eldar and Dark Eldar, Imperial Guard and the Lost And The Damned; to a lesser extent, the Tau Empire and the Imperium of Man. It should be noted, however, that this is considering the overall mentality of the setting, and the aforementioned groups can sometimes be interpreted as the Evil Counterpart and Really Evil Counterpart respectively.

Card Games

 * Magic: The Gathering has several of these. Perhaps the oldest are the White Knight and Black Knight. Though ironically their respective abilities (Protection from Black and Protection from White) means that they can never fight each other.
 * Perfectly symmetrical violence never solved anything.
 * Mirri the Cursed, an alternate universe version of the original Mirri.
 * There's also Crovax, Ascendant Hero, the good alternate-universe counterpart to the original Crovax. Braids gets her own good counterpart in the alternate universe as well. Planar Chaos also gave us a new one, Blood Knight, for Silver Knight, though red is less classically evil than black. Finding examples among cards that only hurt one color is easy, and they can all be considered Psycho Rangers.
 * Sygg, River Guide and River Cutthroat. Actually the same person put through the Lorwyn/Shadowmoor shift.
 * Scars of Mirrodin features a lot of these. You can tell them by the fact that there's a Greek letter phi in the background of the game text.

Tabletop RPG

 * In Nomine has seven major types of angels, and seven major types of demons. Of these types, six types of demons are Evil Counterparts to one of the six types of angels. (Malakim don't Fall, and Lilim are specially created demons with no angelic counterparts.)  For example, the Seraphim are Living Lie Detectors, if they themselves lie, they will eventually Fall, and become Balseraphs, which are Consummate Liar demons.
 * The Villains supplement for the James Bond 007 role-playing game included an urbane and sophisticated rogue CIA agent and serial killer designed as an evil counterpart to James Bond.
 * In Dungeons & Dragons, many of the common races have evil subraces, most notably dark elves and gray dwarves. You'd think that the subterranean deep gnomes are the evil counterparts to surface gnomes, but they're actually True Neutral -- the gnomes' evil rivals are either the spriggans or the kobolds depending on your POV.
 * Also Tieflings (evil counterparts to Aasimars) and the templates fiendish creature (evil counterpart to celestial creature) and half-fiend (evil counterpart to half-celestial). Many spells are evil counterparts too, such as (with their good counterparts listed next to them in parenthesis) Unholy Blight (Holy Smite), Protection From Good (Protection From Evil), and Curse Water (Bless Water). The clerical ability to channel negative energy is the evil counterpart to the clerical ability to channel positive energy, enabling evil clerics and neutral clerics of evil deities to rebuke and command undead (evil counterpart to the ability of clerics who channel positive energy to turn and destroy undead) and replace their prepared spells with Inflict spells (instead of Cure spells as would be the case for those channeling positive energy). The prestige class Blackguard is the evil counterpart to the character class Paladin (in previous editions they were even called "Anti-Paladins") and...oh heck, Dungeons & Dragons could probably justifiably have its own folder for this trope.
 * In 4E, Archons are the elemental servitors of the Primordials and serve as the counterpart to the angels.
 * The Mutants & Masterminds setting of Freedom City has Anti-Earth, home of the Tyranny Syndicate -- expies of the Crime Syndicate. Generally speaking, the Syndicate's members are grade-A douches.