Scale Mail

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

Summary
Issei first activated the imperfect Scale Mail in Volume 2 by making a pact with Ddraig by giving his left hand to the Dragon.

Abilities
The Scale Mail creates a Dragon Armor which increases the user's powers, abilities, speed and defense drastically.

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.

Illegal Move Trident
In later volumes of the light novels, one of the Four Satans, Ajuka Beelzebub, the creator of the Evil Pieces system, remodels Issei's Pawn piece so he can use Promotion anytime and anywhere, regardless if he has already done so. 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 to other forms.
 * Welsh Sonic Boost Knight (龍星の騎士（ウェルシュ・ソニックブースト・ナイト） Werushu Sonikku Būsuto Naito), also known as the Dragon Star Knight, is Scale Mail's Knight form. In this form, Issei possesses enormous speed that far surpasses Sairaorg's speed, but makes his armor thin where it has low defense.
 * Welsh Dragonic Rook (龍剛の戦車（ウェルシュ・ドラゴニック・ルーク） Werushu Doragonikku Rūku), also known as the Rigid Dragon Tank, is Scale Mail's Rook form. In this form, Issei possesses massive power and defense but has a low speed because of its massive armor.
 * Welsh Blaster Bishop (龍牙の僧侶（ウェルシュ・ブラスター・ビショップ） Werushu Burasutā Bishoppu), also known as the Dragon Fang Monk is Scale Mail's Bishop form. In this form. Issei possesses an enormous amount of magical energy in which he applies it to the two cannons at the back of his armor. However, it takes time to recharge once fired.
 * 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. It is equally as powerful as Juggernaut Drive, easily making it the most powerful form of Issei's Scale Mail.

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 that 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.

Imperial Juggernaut Overdrive
Imperial Juggernaut Overdrive (白銀の極覇龍(エンビレオ・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.

Weaknesses
When Issei first achieved 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.
 * 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. His Balance Breaker also gains Dragon wings.

Trivia

 * Issei's Bishop form of his Scale Mail is modeled after the Gundam Double X, the titular Gundam from After War Gundam X.
 * Issei's Knight form of his Scale Mail is modeled after Wild Würger from Namco Bandai's Super Robot Wars series of video games, under Banpresto's Project ATX line of characters. It is also modeled after the titular character in Kamen Rider Kabuto, the 35th installment of Toei's Kamen Rider Series.
 * Issei's Rook form of his Scale Mail is modeled after the titular robot in Sunrise's anime series The Big O.
 * Issei's Queen form of his Scale Mail is modeled after the ZGMF-X10A Freedom Gundam from Mobile Suit Gundam SEED.