 Upgrades common to each game are increased maximum hit points, "sub-tanks" which can be filled with surplus health pick-ups and then used at any time to refill the character's hit points, and the ability to charge weapons earned from bosses, which gives them an enhanced secondary fire mode.In later games, there are multiple armor types available that can either be mixed and matched, or completed for additional armor set bonuses.Starting in "X3", Zero is a playable character.