Zero Gear

Zero Gear (5) (ゼロギア⑤) is a gear that when equipped turns that corresponding person's weapon into their signature weapon; in Roxas's case, he will wield the Kingdom Key+; this also applies to Xion in the multiplayer mode. It's 4 spaces wide, with a square directly below the Gear. It also has room for 4 support pieces.

Weapons
Xemnas = Forbidden

Xigbar = Sharpshooter

Xaldin = Ryūgasen

Vexen = Freeze Pride

Lexaeus = Torn Heaven

Zexion = Tome of Judgment

Saïx = Lunatic

Axel = Eternal Flame

Demyx = Arpeggio

Luxord = Fair Game

Marluxia = Graceful Dahlia

Larxene = Éclair

Roxas = Kingdom Key+

Xion = Kingdom Key+

Sora = Dream Sword

Riku = Way to the Dawn

Donald = Dream Rod

Goofy = Dream Shield

King Mickey = Kingdom Key DΩ

Trivia

 * In Roxas's case, if this Gear is equipped with one Ability Unit, the Keyblade will turn into Oathkeeper. If it's equipped with two, it'll turn into Two Across. If it's equipped with three, you'll turn into Dual-Wielding Roxas with both Oathkeeper and Oblivion, which is only accessible during Mission Mode; in Story Mode, the Keyblade will simply turn back into Kingdom Key+.
 * This gear has the Nobody insignia to signify that each of the weapons this gear equips is a signature weapon of Organization XIII, except in the cases of Roxas, Xion, Donald, Goofy, Riku, and Sora; this is because they are secret characters in multiplayer mode, sans Roxas, and are not part of Organization XIII.