Spirit Partner

Spirit Partners are Duel Monster Spirits that have human partners. Their partners and any other humans that possess the ability are the only humans who can see and interact with them. It's also possible to have more than one spirit partner. The following characters own or have the ability to sense a Duel Monster in one way or another. They are presumably Ka due to how Winged Kuriboh appeared in a flashback in the manga.

Yu-Gi-Oh!

 * Yugi Muto has been shown the ability to communicate with spirits, though the ability was rarely touched upon. His Spirit Partners appear to be Dark Magician, Kuriboh, and Dark Magician Girl. He may also have some bond with Maha Vailo, as he said this monster is the most like him.
 * In Yu-Gi-Oh! GX, he is shown to communicate with Winged Kuriboh.


 * Seto Kaiba appears to have Blue Eyes White Dragon as a Spirit Partner. The spirit told him it wanted to fight during the Duel against Ishizu. Kaiba was even willing to sacrifice a God card in order to bring out his loyal servant.


 * Rafael was able to see the spirits of his "Guardian" monsters when he lost his Duel to Atem, but this was not fully explained. His Spirit Partner appears to be Guardian Eatos. When he is controlled by "The Seal of Orichalcos" his Spirit Partner is Guardian Dreadscythe.


 * Joey Wheeler's spirit partner appears to be Flame Swordsman as it was his Deck Master and he actually turned into him in the Duelist Kingdom Saga in a Shadow Game against Bakura.


 * Téa Gardner Spirit Partner appears to be the Magician of Faith as she bonded her soul to the card in Episode 13, Yugi combines some of Joey's, Téa's and Tristan's cards into his Deck against Bakura/Yami Bakura.

Yu-Gi-Oh! GX

 * Jaden's Spirit Partners are Winged Kuriboh and Yubel. Later on, he's also able to verbally communicate with the Neo-Spacians as well as Elemental Hero Neos and Honest.


 * Chazz's main Spirit Partner is Ojama Yellow in the anime, and Light and Darkness Dragon in the manga. However, he is also able to see the other two Ojamas that make up the The Ojama Trio, The Dark Scorpions, as well as all the monsters from the well.


 * Jesse's main Spirit Partner is Crystal Beast Ruby Carbuncle. He’s also able to interact with the rest of the Crystal Beasts plus the ones floating around such as Tom’s Jerry Beans Man. He also considers all of his monsters his family.


 * In the GX manga, Koyo Hibiki is the original owner of Winged Kuriboh before passing it down to Jaden. He too is able to communicate with his Duel Spirit. Midori, his sister, is also able to see and hear that Spirit.


 * Chancellor Foster was the original owner of Ojama Yellow before he passed it to Chazz. He could see and hear it, but it is unknown if he could do it with other Duel Spirits.


 * Belowski can communicate with Duel Spirits like Jaden. His spirit partner may be Mokey Mokey.


 * Yusuke Fujiwara's partner used to be Honest before he was possessed by Nightshroud.


 * Trapper doesn’t possess a Spirit Partner of his own (he once did, but it was stolen from him by another, unnamed person), but nonetheless, wields the uncanny ability to not only see, but also trap Duel Spirits.


 * Chumley has Des Koala as his Spirit Partner. It's unknown whether he can see other Duel Spirits, but he is at least able to hear them.


 * After his Duel with Jaden, Aster Phoenix began to see "strange things", which the former interpreted as Duel Monster Spirits. It is known that Aster can sense them and see their shadows. He may not have a Spirit Partner, but it is to believe that his Spirit Partner is Destiny Hero - Dreadmaster.


 * Zane's Spirit Partner is Cyber End Dragon. In episode 83, it was revealed by Sheppard that he could hear the monster's Spirit and Zane later stated that he could hear it crying out in pain after destroying it.


 * Princess Rose claimed to be able to see Duel Spirits; though no one believed her, Jaden saw a Des Frog in a Prince's outfit following her after their Duel, leading viewers to believe she was telling the truth.


 * Mr. Stein has the ability to only see his own monster, Scab Scar-Knight/Scar-Knight.


 * Though Sartorius can not see visually see nor speak to Duel Spirits, he is able to sense them due to his honed psychic abilities.


 * According to Wheeler's doctor, monkeys have higher ability to hear Duel Spirits than humans. That's why he made Dueling experiments with Wheeler.


 * Yubel may be considered a Spirit Partner, though the relationship she shares is mostly harmful to all who come into contact with her, so this is debatable. Notably, however, she does afford great power to those who ally with her, although she often turns on her allies when she has no further use for them. However, after fusing with Jaden and being freed from the Light of Destruction's influence, Yubel actually became a loyal spirit partner to Jaden.

Yu-Gi-Oh! 5D's

 *  Luna's spirit partners are Kuribon and Ancient Fairy Dragon, though she can see the spirits of the rest of her deck, all of which are spirit monsters such as Regulus and Sunlight Unicorn. She can also speak to the spirits in other Duelists' decks, whether or not that duelist can see or speak to spirits or not.


 * Leo appears to have "Power Tool Dragon" as a spirit partner, as his dragon has often shown concern for him in his duels. Leo even asked if it was okay in episode 49, and it nodded, showing that the two can understand each other (whether or not they can mentally communicate like Luna outside of a Duel, is yet to be seen). He later revealed himself in his true form, Life Stream Dragon.


 * Yusei Fudo appears to have "Stardust Dragon" as a Spirit Partner, as Luna says in episode 41 "Is Stardust saying he'll protect us?" (when she says this, Stardust Dragon moved a little more in front of them on its own).

Yu-Gi-Oh! ZEXAL

 * Astral's and Yuma Tsukumo's spirit partner is Number 39: Utopia as he is the Number most loyal to Astral and Yuma. This is seen in episode 37 when Utopia protected Astral on its own, and in the episode 39 even left the key because of Astral telling him to go to Yuma. Utopia also left the key in episode 74 to go help Yuma against Fuma.


 * Baby Tiragon has also been shown to interact with Yuma like a spirit partner.