Starting in "X3", Zero is a playable character. In "X3", Zero is armed with a beam saber in addition to the traditional "buster" arm cannon, but relies on his saber almost exclusively starting in "X4". In "X5", both X and Zero gain the ability to duck. In "X6", the player can rescue reploids to replenish health and acquire upgrades not otherwise available. In "X7", the playable character Axl is introduced. Axl utilizes two guns known as Axl Bullets. In "X8", a tag system is introduced, along with a new Double Attack feature, where the two selected characters can attack at the same time.