Zero's Forms

Similiar to X's Armor System, Zero is also able to change into more powerful forms.

Power Up Parts
Similar to X, Zero can use power up parts in Mega Man Xtreme 2. Each parts upgrade his skills when he enters in Dr. Light capsules.

Black Zero
Black Zero (ブラックゼロ) is a hidden version of Zero appearing in Mega Man X4, X5 and X6, as well as Mega Man X8 and Command Mission. It usually appears as a black colored version of Zero with grey to white hair and a recolored Z-Saber blade. While it is purely a recolor in X4, it features several abilities during its following appearances. In X5 and X6, it increases Zero's attack and defense capabilities. In X8, the Zero Armor appears once again; however it is more balanced than its previous incarnations. It still doubles Zero's attack strength but at the same time decreases his defense power by half. Finally, Black Zero is available to Zero right from the start in Command Mission as his Hyper Mode, boosting all of his stats. Except in Command Mission, the Zero Armor has to be unlocked by either a cheat code put in at the game's title screen or by clearing certain objectives (such as finding Dr. Light's secret capsule in X5 or obtaining all Rare Metals for Zero in X8).

Absolute Zero
Absolute Zero (アブソリュートゼロ) is an additional Hyper Mode for Zero obtainable in Mega Man X Command Mission. It can be gained by defeating the secret boss Duckbill Mole located in the Melda Ore Plant. Its design is often compared to that of Bass from the original Mega Man series when combined with Treble due to its color scheme and devil like wings. Aside from this, it boosts all of his stats vastly, even doubling Zero's attack power, and have the ability to break through any defense. He does not usually use weapons in this form, since he can use his claws in powerful melee moves. His Main Weapon becomes the attack Crimson End (クリムゾンエンド), and his Sub Weapons the Water-attribute attacks Killer Left (ジェノサイドレフト) and Brutal Right (ダムドライト). His Action Trigger becomes Calamitous Arts, which makes a DDR-style screen come up, and commands must be entered in. Missing one will not perform that specific strike. The total time in which the Action Trigger lasts is determined by the amount of WE available.

Rockman X Mega Mission armors
In the Carddass Rockman X Mega Mission, Zero got a Neo Buster (ネオバスター) when he went to rescue X in Mega Mission 2, who was captured by a group known as Order Breakers. After defeating Magna Centipede L, he receives the Energy Armor (エネルギーアーマー) from Dr. Light, which only has a body part and gives more power to his buster. It later upgraded to the Neo Armor (ネオアーマー) after absorbing energy in the fight against Spark Mandrill L, with the body part modified and the appearance of leg parts. After rescuing X, they fought against Tackione, the leader of the Order Breakers. Damaged because of the battles, Zero gave the Neo Buster, Neo Armor and his saber to X for him defeat Tackione's final form, becoming the Giga Armor.

In Mega Mission 3, Return X was shot in his back by Enemice HL, and to fight him he fused with Zero and X to improve their power, becoming the EX Armor (イクスアーマー Ikusu Āmā), which has a buster and body part. When Zero was knocked out, the part of Return X that was on him moved to X, improving his armor.

Form System
Appearing in Mega Man Zero 2, the various forms are unlocked by completing missions under certain circumstances and grant Zero special abilties. Note that only one form can be obtained per mission. The Proto Form and Ultimate Form are also available in the game Onimusha Blade Warriors, but are depicted as the X and Energy Forms.

Trivia

 * The Proto Form from the Zero series is generally assumed to be the Black Armor, having the same statistics of the armor from X8, but with some added handicaps, notably the inability to use the Z-Saber combo.