Scale Mail

The Scale Mail is the Balance Breaker armor of Issei's Boosted Gear and Vali's Divine Dividing.

Summary
Issei first activated his imperfect Scale Mail in Volume 2 by making a pact with Ddraig by giving his left hand to the Dragon. When Vali appeared in Volume 3, he was in his Balance Breaker form, a perfect version of the Scale Mail. In Volume 4, when Issei fought against Vali, Issei used a bracelet given to him by Azazel which allows him to use the imperfect Scale Mail for 20 Minutes. Issei achieved his perfected version of the Scale Mail in Volume 5 after poking Rias' breast. After gaining the Scale Mail, it became the primary battle form for Issei and Vali.

Abilities
The Scale Mail creates a Dragon Armor which increases the user's powers, abilities, speed and defense drastically. It also temporarily allows the user to use the power of the Boosted Gear and Divine Dividing without the 10-second time limit.

In Volume 9, Issei caused a cannon to form on one of his gauntlets, suggesting that the armor may be altered to fit the will of the wearer.

In Volume 11, it is suggested that the Scale Mail can (at least temporarily) protect the wearer, and allow them to survive within the Dimensional Gap.

Weaknesses
When Issei first attained his Balance Breaker, it had the following weaknesss:
 * Activating Balance Breaker takes two minutes, during which a countdown begins on the gauntlet gem during this period. He cannot cancel the process, or use other abilities such as Boost or Transfer.
 * It could only be used for half an hour once a day, and his Sacred Gear becomes powerless the rest of the day.
 * In Scale Mail mode, Boost and Gift cuts down 5 minutes of Balance Breaker per use.
 * After using Juggernaut Drive, Issei's Scale Mail countdown is reduced to 30 seconds, can be used up to 2 hours (3 on a good day), and can activate multiple times a day as long as those two hours have not been used up yet. His Balance Breaker also gains Dragon wings.

Boosted Gear Scale Mail
The Scale Mail of Boosted Gear used by Issei creates a Red Dragon Armor with green jewels that covers the body of the user. In this form, Issei can double his power any time he wishes, but at the cost of a large amount of stamina and magical power.

Illegal Move Trident
In Volume 8 of the light novels, Ajuka Beelzebub, one of the Four Satans and creator of the Evil Pieces system, remodels Issei's Pawn piece so he can use Promotion anytime and anywhere. During Promotion, Issei can now use Illegal Move Trident (赤龍帝の三叉成駒（イリーガル・ムーブ・トリアイナ） Irīgaru Mūbu Toriaina) to change his Scale Mail armor into other forms.

Cardinal Crimson Promotion
Cardinal Crimson Promotion (真紅の赫龍帝（カーディナル・クリムゾン・プロモーション）Kādinaru Kurimuzon Puromōshon), also known as the True Crimson Dragon Emperor, is Scale Mail's Queen form that debuted in Volume 10 of the light novels during Rias' Rating Game battle against Sairaorg. It changes the color of Issei's armor to crimson. Cardinal Crimson Promotion incorporates all three Evil Pieces traits, making it the most balanced and most powerful form of Issei's Scale Mail. However for full use, it requires all Illegal Move Trident forms to be fully developed and mastered before it can be completely stabilized, and its full power brought out.

It is stated to be Issei's answer to an alternative for Juggernaut Drive, and even more powerful than it.

Super Gigantic Balance Breaker
Super Gigantic Balance Breaker (超巨大禁手) is a form Issei attained in Volume 12 by gaining the Great Red's help, in which Issei reproduces the size of the Great Red that he uses to destroy the beast created by the Annihilation Maker. In this form, he gains highly destructive powers such as the forbidden and sealed power Longinus Smasher, which can cause an area in the Underworld to disappear without a trace.

Divine Dividing Scale Mail
The Scale Mail of Divine Dividing used by Vali creates a White Dragon Armor with blue jewels that covers the body of the user.

Empireo Juggernaut Overdrive
Empireo Juggernaut Overdrive (白銀の極覇龍(エンピレオ・ジャガーノート・オーバードライブ Enpireo Jagānōto Ōbādoraibu) is a new form attained by Vali in Volume 12. It changes the color of his armor to silver. Like Issei's Cardinal Crimson Promotion, it does not consume his life.

Trivia

 * Cardinal Crimson Promotion is modeled after the ZGMF-X10A Freedom Gundam from Mobile Suit Gundam SEED.