"Shadi" (شادي) is a name used in Arabic-speaking countries as well as in the Indian subcontinent. The name is traditionally given to Arab Christian and Muslim males. The word "Shadi" is of Arabic origin and has the archaic meaning of "singer" although that is no longer its use nowadays.
Shadi was killed by Bandit King Bakura at least 3,000 years (5,000 years in the english dub) before the present day events of the series, meaning that Shadi is a disembodied spirit and insubstantially incorporeal ghost normally whenever the characters encounter him. However, such as when saving Tristan and Duke's lives from falling off Kaiba's blimp, Shadi does have the power to become substantial for a short time. It is implied in the anime that he is actually the spirit of Shada combined with that of Karim, as Karim had given his life-force to Shada so that he may continue to protect the Pharaoh, which explains why Shadi holds both the Millennium Key and Millennium Scale.
Shadi had guarded the tomb where the seven Millennium Items were once kept. Shadi forcefully gave the Millennium Eye to Maximillion Pegasus, after Pegasus saw him punish a grave robber with the Millennium Ring. Shadi was the apparent guardian of the Tomb.
When Yami Marik murdered his father, Shadi gave him the message that the Pharaoh would return, causing Marik to believe that the Pharaoh was responsible for his father's demise. In the English version, the father was sent to the Shadow Realm.
Shadi appeared before Yugi in Duelist Kingdom and entered his mind. Shadi found that Yugi's soul had been divided into two soul rooms. One of them was for Yami Yugi and the other was for Yugi. Shadi entered Yami's mind to find any clues that might lead him to the thief who stole Pegasus's Millennium Eye (the thief was Yami Bakura). As Shadi attempted to find any clues, he was attacked by multiple traps in the Pharaoh's mind. Shadi almost met his end but was rescued by Yugi. As both attempted to find clues to the thief of the Millennium Eye, they were attacked by Dark Magician who emerged from a stone tablet. Shadi was standing on a stone tablet of the Blue-Eyes White Dragon and attempted to defend him and Yugi by summoning the dragon. However Yugi reasoned with the Dark Magician and convinced the magician to call off his attack, much to Shadi's amazement. Shadi then suspected that Yugi is the chosen one. Once Shadi exited Yugi's mind, he apologized for the intrusion. He then warned Yugi to be careful and watch out for the thief. Before he dispersed, Yugi asked for his name and Shadi told him.
Shadi appeared once again during Battle City; he appeared saving Tristan and Duke, whom had gotten into a fight over Serenity and forced to hang onto the end of the blimp for their lives.
Later, he appeared to Yugi and warned him about all seven Millennium Items being aboard the Kaiba Corp blimp used during the Battle City tournament. Then, he explained to Yugi how Pegasus came across the Egyptian Gods and recreated them as Duel Monster cards. He then told him of the curses that came afterwards which caused the death of everyone involved in the production of the gods (whereas they vanished without a trace in the English version), and that Pegasus was forced to return them to the tomb where he discovered the gods. Shadi then met up with Ishizu Ishtar to discuss the evil within Marik Ishtar that was growing (Yami Marik) and must be stopped. He then vanished once again.
Shadi helped Yami Yugi during the Dark RPG. He appeared in front of the stone tablet which bears the image of Priest Seto and Pharaoh Atem. Tristan was able to recognize him and pointed out that Shadi was the one who saved him and Duke during the Battle City tournament. Using the power of the Millennium Key, Shadi got Yami Yugi's friends into the Millennium World through Yami Yugi's memory. When Pharaoh Atem was about to be killed by Zorc Necrophades, Shadi possessed the body of Hassan [from Arabic حسان or حسن (depending on pronunciation)] to help Atem during his fight with Zorc. However, he met his destruction with a blast from the evil being.
After the Dark RPG and the Ceremonial Battle, Atem proceeded to his afterlife, and the seven Millennium Items were destroyed. While the tomb was crumbling and the people had left the main chamber, Shadi's spirit had appeared and disappeared one last time; finally able to rest in peace in the spirit world.