The Dark counterparts are a series of DARK monsters that is identified by an existing non-DARK counterpart and 「ダーク」 in their Japanese name. Dark counterparts also have a twisted, sinister appearance compared to their original forms, which can be especially striking if the original form was that of a benevolent monster. Because of this Dark counterparts share the same ATK, DEF and Level with their non-DARK counterpart.
The Dark counterparts were released in the OCG and TCG in Phantom Darkness, but dated back to the anime with "Rainbow Dark Dragon". The TCG exclusive Dark counterpart cards were introduced in OCG in the Extra Pack.
Dark counterparts have no external support but are highly playable and powerful, particularly "Dark Armed Dragon", which supported one of the most competitive tournament Decks around (Tele-DAD). However, the March 2009 banlist severely hampered this Deck, with some of the main cards, such as "Emergency Teleport", "Destiny HERO - Malicious" and "Plaguespreader Zombie" (not to mention "Dark Armed Dragon" itself) being Limited and Semi-Limited, making it somewhat obsolete.
Dark counterparts usually have effects similar to their counterparts (with the exceptions of "Dark Grepher" and "Dark Valkyria", whose counterparts are Normal Monsters), but reworked to go with the theme of manipulating DARK monsters you own or control, typically either summoning them, sending them to the Graveyard, or removing them from play (from the Graveyard) to trigger effects.
Although they do not explicitly support Dark counterparts, several cards released in Phantom Darkness work very well with them. "Doomsday Horror" gains power from removed DARK monsters and can "reload" your Graveyard with the removed DARK monsters. "Escape from the Dark Dimension" can Special Summon one of the removed DARK monsters, and "Allure of Darkness" increases your draw power while removing another DARK monster. "Veil of Darkness" can also vastly increase a DARK-user's draw power while flooding the Graveyard with DARK monsters to use as a resource.
Individual Dark counterparts splash well into a number of other established archetypes as support cards. They work best with Archetypes that focus on Graveyard control and which are composed of DARK monsters, such as "Destiny HERO" monsters, Zombies, and Fiends.
There is some debate as to whether "Arcana Force EX - The Dark Ruler", "Malevolent Mech - Goku En", and "Paladin of the Cursed Dragon" should be included in the categorization of this series. They are clearly counterparts of "Arcana Force EX - The Light Ruler", "Majestic Mech - Ohka", and "Paladin of White Dragon", but despite their names, have the same Attribute (LIGHT). All of them have 「ダーク」 in the katakana part of their Japanese names, rather than the kanji/furigana. The latter two are considered Zombie counterparts.
The Japanese name of "Darklord Zerato" does not contain 「ダーク」, and its name in the OCG is "Fallen Angel Zerato". However, this card is still considered to be the Dark counterpart to "Archlord Zerato" by some players, even with the pre-existent "Mazera DeVille".
"Adreus, Keeper of Armageddon" is seen as the Dark counterpart of "Tiras, Keeper of Genesis", both cards sharing rank, ATK, DEF and having a similar name. However not having "Dark" in its name and being of a different type (Adreus being a Fiend and Tiras a Fairy) result in Adreus not being placed in the series.
"Shadowpriestess of Ohm" is occasionally placed within this series, since she is a DARK monster with an effect related to other DARK monsters and bears a strong resemblance to another existing card ("Lady of Faith" and "Elemental Mistress Doriado"), as well as having been released in Phantom Darkness alongside many of the Dark counterparts. However they possess differences in ATK and level, and as a result, she is not a Dark counterpart.
"Darkflare Dragon" is often considered a Dark counterpart to "Lightpulsar Dragon", since it is a DARK monster and has "Dark" in its name. However they possess differences in ATK, DEF and level, and as a result, it is not a Dark counterpart. Also, judging from the story of Dragons Collide in the official site, Darkflare Dragon is more of a companion or comrade to Lightpulsar Dragon, than a counterpart.
"Dark Elf" almost fits, as well, as it is obviously a Dark counterpart to "Mystical Elf" with practically the same design. The key difference is that the ATK and DEF of "Dark Elf" are inverted from "Mystical Elf".
Trap cards that may fit are "Dark Mirror Force" and "Dark Trap Hole". For the former, the effect is somewhat reversed from the original and with the latter the effect is just watered down. However, the effect has nothing to do with DARK monsters.
Archetypes composed of DARK monsters based on previous monsters, like "Malefic" or "lswarm", are not considered part of the series because they lack the attributes that compose it (like sharing the ATK, DEF and Level with their non-DARK counterpart, or having "Dark" in their name). Instead those archetypes' attributes and appearance transform their members into different versions of the original card, like Corrupted versions ("Malefic") or infected versions ("lswarm").
The "Lightray" Series can be thought of as the inverse of Dark Counterparts, as they represent LIGHT versions of existing monsters that exhibit synergy. "Warrior Dai Grepher" has both a DARK counterpart and a Lightray counterpart.