Codebreaker (「コードブレイカー」 Kōdobureikā) is an archetype of DARK Warrior monsters used by Cal Kolter in the Yu-Gi-Oh! VRAINS anime and first debuted in Eternity Code.
The members of this archetype are based on computer-software vulnerability terms:
The "Codebreaker" archetype focuses on being detrimental to Link Monsters other than their own, primarily through stat reduction and potentially gaining more devastating effects depending on how many of them are present on the field. "Zero Day" is the main way they accomplish this, as any monster - bar a "Codebreaker" - loses 1000 ATK for each copy of it linked to that monster. Naturally, this can result in a large portion of a monster’s ATK disappearing and allowing it to be destroyed in battle much easier. But if a "Codebreaker" is destroyed while "Zero Day" is on the field, all copies of the latter are immediately destroyed as well; to mitigate this, the monster is able to add other copies from the Deck to the hand. Their first Link Monster, "Virus Swordsman" also helps with this as it’s able to Special Summon a "Zero Day" from almost anywhere, which can also possibly happen during either player’s turn as this activates whenever the monster is Summoned though it must be co-linked at the time. It can also revive itself during the end of a turn itself was destroyed but will be banished if it leaves the field. Their second Link Monster, as well as their last monster entirely, is "Virus Berserker"; upon being Special Summoned "Berserker" can revive any pair of "Codebreaker" monsters to any zones a Link Monster points to. It can also destroy Spells/Traps, up to the number of other members being linked.
Due to being DARK Warrior monsters, cards like "Reinforcement of the Army" and "Allure of Darkness" can help immensely. One should also consider cards like "Summon Limit" to further dampen the opponent’s plays, potentially causing them to be stuck with weak monsters.
The main strategy of the "Codebreaker" archetype is the fact one is effectively giving the opponent fodder for their Extra Deck, especially since the monsters lack any ability to lockout any monsters from there as well the monsters still being perfectly good Tribute fodder. This is especially bad due to powerful monsters generally requiring more materials for their Summons or effects such as "Monarch" monsters and particularly strong Link Monsters such as "Saryuja Skull Dread". "Shadow-Imprisoning Mirror" may also prove to particularly devastating.