QUARTET

[ QUARTET ] is a Special Keyword featured in some Level 1 monsters.

If a monster has the keyword [ QUARTET ], it means that you cannot have more than 4 of that type of monster. It is a limitation in Deck Building, as there are several monsters that are either too strong, or too universal to be added at multiple copies (more than 4) in several decks.

[ QUARTET ] itself has 2 sub-types: [ GUARDIAN ] and [ RAPID EVOLUTION ]. The former lets you negate a direct attack and even ending the Battle Phase, giving you not only an extra creature (useful for Summoning an Evo Monster too), but a free turn without damage. The latter, however, lets you immediately Extra Summon a monster from your Extra Deck (an Evo Monster) and giving you an instant advantage over your opponent.

[ QUARTET ] cards will not have their own unique evolutions.