Faust stole the Pandora Box as well as a few Fullbottles from Build! Night Rogue does not back down as he aims to claim all of the Fullbottles left in Build's possession. The only ones left behind are the Lock and Dragon Fullbottles. The KeyDragon form can only be accessed by Build, however, the power within the Dragon Fullbottle is so strong that it becomes impossible to control even in a Best Match configuration! In desperate circumstances, there doesn't seem to be much of an alternative, until Sento thinks of one... if the Dragon Fullbottle can be used in the Cross-Z Dragon, it will become compatible with the Build Driver! (Source: kamenrider.fandom)