Hasan (Arabic name 'حسن': means good in look) is a mysterious ancient being and is the messenger of the Egyptian Pharaohs. For centuries he has defended the honor of Egypt's Kings, and he made a vow with Atem's father Aknamkanon to protect his son no matter what.
During the Dawn of the Duel arc, Hasan makes a first appearance in Ep.208 as a mysterious figure which appears to Pharaoh Atem inside a cave by the river Nile. His identity is then fully revealed in Ep.211, and his role in the Shadow RPG explained: he is Yami Yugi's "Master Item" (In the anime only; in the manga he is an NPC).
When Zorc is about to blast Atem, Hasan appears and blocks the attack only to have his helmet broken, revealing him to be Shadi. Zorc's blast is believed to be too powerful for Hasan to sustain, and he is vanquished. (this only happened in the manga. In the anime, Zorc killed him, then Shadi possesses him). Zorc's blast is believed to be too powerful for Hasan to sustain, and he is vanquished.
In the manga, Hasan is really Bobasa and Shadi. He is also the spirit of the Millennium Stone, in the manga.