GUARDIAN

[ GUARDIAN ] is a Special Keyword and a Sub-Type of [ QUARTET ] cards that lets you Extra Summon a monster during your opponent's turn, when you are attacked directly by one of your opponent's monsters. They are not only stopping that attack, but also finishing the Battle Phase itself, giving you a free turn to not only recover from your opponent's attacks, but also to Summon your own monsters. The monster Summoned by [ GUARDIAN ] even lets you use the monster Summoned as a material for the Extra Summon of an Evo Monster.