Duel Monster Spirits, Duel Spirits, or what are now known as Duel Monsters is the name given to the magical, spiritual, and otherworldly creatures who have the capability to be summoned by a duelist. Ancient legends and carvings of Duel Monsters inspired Maximillion Pegasus to create the popular card game of the same name. They live primarily in the Duel Monsters Spirit World, although their relationship with humans and Earth are sometimes unclear, and differ within the different Yu-Gi-Oh! series. It is unknown if every Duel Monsters card created within the Yu-Gi-Oh! universe has a Duel Spirit counterpart that can be summoned accordingly.
In some cases, these spirits seem to form a strong bond with a specific human. In Ancient Egypt, these spirits took the form of a person's Ba (In Egyptian mythology, the Ba was the other half of your soul - your personality). In Egypt, some people could possess multiple spirits as their Ba. A person's Ba can fuse with their Ka (life energy) to fuse the person with their spirit, such as when Mahad became Dark Magician. It is unclear how this relationship takes form in the modern age, and in each other Yu-Gi-Oh! series - for example, the Ba of each Signer could be their Signer Dragon, but it is never stated. When a person dies, they can either pass on into the afterlife, or fuse with your Ba to become a Duel Monster Spirit (Like Mahad and Mana). It is highly implied that the afterlife and the Duel Monsters Spirit World are both connected somehow.
These spirits have their own societies in each part of the spirit world (the spirit world is explained by Yubel to be several entirely different dimensions), and there have even been conflicts between the spirits of some dimensions, such as between DARK Fiend-Type monsters against LIGHT Fairy-Type monsters or Warrior-Type monsters against Dragon-Type monsters, among others. Duel Spirits were allies of the human world in many ancient societies such as Atlantis, Egypt and the People of the Stars. The Earth has been both aided and plagued by both good and evil spirits.
One section of the spirit world appears to be based of mostly Fairy and Dragon type monsters although Dark Magician Girl appeared to be the leader of them. Her presence in this place may indicate other Spellcaster type monsters such as Dark Magician also live there. The most notable Duel Spirits from this area are the Legendary Knights of Atlantis, Timaeus, Critias, and Hermos who are it's guardians. According to Dark Magician Girl, the spirit world has existed as long as mankind has existed.
Another section is Neo Space, which is the realm beyond Earth in outer space in which consists of many Alien duel spirits such as Elemental Hero Neos and the Neo-Spacians. They believe in a being with the power to bridge the worlds of the Duel Spirits and the humans called the Supreme King, whose destined to save both worlds from The Light of Destruction.
Another unique section of Duel spirits are the Numbers, whose powers originate from both Astral and Barian Worlds. Even several spirits were chosen to act as guardians to protect their respected Number cards on Earth.
Duel Spirits have played a pivotal role in human history dating back at least ten thousand years. Humans somehow found a way to summon them here the human world using magic and spells. The Ancient Egyptians sealed their spirits (or the means to summon them) in stone tablets by ripping a person's Ba from them, killing them in the process. These tablets resembled duel monsters cards and later inspired Maximillion Pegasus to create the game. In doing so, Pegasus sealed the duel spirits (or means to summon them) in the cards, similar to how they were sealed in the tablets. However, they are only truly summoned when magic forces are involved or when they are involved with certain individuals. Otherwise, they are merely represented by holograms projected by KaibaCorp technology. Therefore while they have true physical form in their own world, they cannot take physical form on Earth without being summoned here. Duel Monsters Spirit Day is an exception however being the only day of the year when Duel Spirits can take physical form on Earth without elements of magic being involved.
Back in the times of Atlantis, the spirits came to the aid of Ironheart, Chris and the survivors of Atlantis after its king, Dartz, corrupted and brainwashed by the Orichalcos, began a conquest to destroy all life on Earth to recreate it. They sent a huge army against Dartz's Orichalcos soldiers and the Great Leviathan in the Battle of Atlantis. Neither side was victorious, Atlantis sank into the ocean depths and the Duel Spirits were sealed away until they be needed again.
Around that time the Crimson Dragon and its servants the Signer Dragons battled the evil entity called Red Nova to exhaustion. It was only through the help of a Legendary Signer that Red Nova would be defeated and sealed away into the Earth and become the first of the Nazca lines, thus becoming the first Earthbound Immortal.
Regardless of their nature, Duel Monster Spirits seem to possess different powers depending on who the monster is. For example Jinzo and Yubel have the ability to possess people and take on a physical form if they have enough energy. Winged Kuriboh could take Jaden Yuki to another dimension. Brron was capable of performing a ritual using negative feelings to create "Super Polymerization", to name a few.
A common trend however is that seemingly all Duel Spirits can Duel themselves and have their own Deck, which in most of not all cases will have at least one copy of the card they themselves represent, regardless of rarity. For example in Yu-Gi-Oh! GX the spirit of Dark Magician Girl had her own card of "Dark Magician Girl" even though Bastion Misawa claimed Yugi Muto was the only known owner of the card.