Emperor's Key

The Emperor's Key (皇の鍵, Ō no Kagi), sometimes shortened to the Key in both versions and also called the Golden Key in the dub, is a pendant belonging to Yuma Tsukumo.

History
Kazuma Tsukumo received the Emperor's Key from a being similar to Astral. This caused him to see an attack occurring in the Astral World. Kazuma later passed to key on to his son, Yuma.

Yuma held onto the key as a keepsake of his traveling parents and usually wore it around his neck. He felt that the key opened many possibilities and made him think he could take on any challenge. Despite this, Yuma suffered nightmares about a demonic door, telling him that whoever opened it would receive great power at the cost of a great risk. The key would light up during these dreams.

Yu-Gi-Oh! ZEXAL
One day, Yuma confronted Reginald "Shark" Kastle as school, over him taking Bronk Stone's Deck through an ante. During the encounter, Shark noticed Yuma's fondness of the key, so he swiped it. He proceeded to break it in front of Yuma and throw a piece away. Bronk managed to find the piece Shark threw away and later returned it to Yuma.

The next day, Yuma faced Shark in a Duel. Before they began, Yuma held up the piece of the key Bronk returned to him, and as he demonstrated his confidence, the other part of the key was transported from his house to his hand and the key repaired itself. Yuma was transported to the door from his dreams and this time, he inserted the key. The door was unlocked releasing Astral. Yuma then returned to his Duel, with Astral by his side.

The granted resistance to Numbers when Number 96: Dark Mist tried to take "Utopia" from Bronk Stone by force, and when Bronk Summoned "Utopia". When Shark had the Key and took control of Kite Tenjo's "Acid Golem". However, the Key was unable to fully suppress the power of a "Chaos Number" when Yuma took control of "Number C32: Shark Drake Veiss". Though the "Chaos Number" did not possess him, the connection to it weakened Yuma and surrounded him in a dark aura which was eventually electrified, causing him great pain. Vetrix pointed out that if Shark did not do something, Yuma would be taken over by the "Number" like he was.

World Duel Carnival


The original owner of Key, Kazuma Tsukumo, also has power over the Key as well. He mysteriously appeared in the Key and used the power of the 12 collected "Numbers" to break Trey's influence over Yuma, allowing him to regain his "kattobingu". Kazuma later returned to tell the revived Astral that his son is calling him for help.

Yu-Gi-Oh! ZEXAL ll
When Yuma Dueled against Fuma, Astral (whom was in the key) was electrocuted at times when Yuma's Life Points went down.

Manga
The Emperor's Key fell into the hands of Mirai Tsukumo, who passed it on to her son, Yuma.

Features and functionality
The Key allows Yuma to bring Astral forth, which is similar to that of when Yugi Muto turns into Yami Yugi. Astral can go into the Key and spend most of his time in a pocket dimension. In case the Key is taken from Yuma or Yuma loses it, Astral will be trapped inside the Key if he is within it. However, that won't be the case if he is outside of the Key.

There is a structure in the Key, which, according to Astral, is some sort of spaceship. Inside the ship's construction is a puzzle, which is made up of hundreds of gears marked with numbers that react with the "Number" cards that Astral absorbs, making them move and glow. When a "Number" appears on the machine it starts turning on its pieces. Astral assumed that it had to do with the person who is responsible for the loss of his memories. The "Number" cards also appear to lock into the structure and become apart of it, which keeps them safe. The "Numbers" are printed on square pillars that stick out and glow, but if the Key is separated from Yuma's Deck then the pillars go back into the structure and the "Numbers" become cards again meaning they are in danger of being taken.

The Emperor's Key gives the user some resistance to the Numbers' influence. It has at least once failed to fully suppress the power of a "Chaos Number", "Number C32: Shark Drake Veiss", causing the user great pain.

A diabolical door has haunted Yuma in his dreams trying to convince him to open it. The Emperor's Key was also the key to open that door, which released both Astral and the "Numbers"; and to do so, the broken key restored itself. When the Key got separated from Yuma, a voice said that "Astral is immersed in the guidance of time" and that he must obtain the "power of ZEXAL". The voice described it as a power that turns all darkness into light. The Key has power to release the "Number's" "alternate form", "Chaos Number".

The "Numbers" have power inside the Key, as when Astral sent "Number 39: Utopia" off, the Key fired the card out of the Key and into Yuma's Extra Deck.