|Corresponding card||Mystical Elf|
|Japanese translated||Princess Mary|
|Other names||Mystical Elf|
|Title||Princess of Sin Lau|
|Anime debut||Yu-Gi-Oh! episode 04444: "Legendary Heroes, Part 2"|
|Japanese voice||Eri Sendai|
Princess Adena, known as Princess Mary (メアリー姫, Mearī Hime) in the Japanese version, is a character in the Yu-Gi-Oh! anime. She is the princess of Sin Lau, a kingdom in KaibaCorp's Legendary Heroes game. Her design was created by the Big 5, based on Seto Kaiba's little brother, Mokuba Kaiba.
In the English dub, Adena has a British accent. She is extremely well-mannered and kind.
Princess Adena met Yugi Muto, Joey Wheeler, Mokuba Kaiba, and Mai Valentine in a maze, when Earu led them to her. After they defeated "Gate Guardian" Adena mentioned that they were like the heroes of legend. She took them to her palace and told them about the suffering of her people. She said that they must make two sacrifices to resurrect the "Mythic Dragon". She added that one offering was an unknown stranger (which was Seto Kaiba), and that she has volunteered as the other offering. Adena mentioned that there was a legend that once, a hero escaped the "Castle of Dark Illusions" on a flying machine; the flying machine was never found and that there was no other way to reach the castle. Mai commented that they could fly there on "Red-Eyes Black Dragon". Adena sadly explained that a magic barrier protected the castle, and that no Duel Monsters can reach it. She said that the legend had a prophecy, that brave heroes will destroy the "Mythic Dragon"; Adena also said that Yugi, Joey, Mokuba and Mai were the heroes. Lightning struck and everyone went outside with Joey, Mai, Yugi, and Mokuba dressed in battle armor. Suddenly, a net thrown by "Armed Dragon" came down and took Adena. But it turned out that it was Mokuba, dressed as Adena. Joey, Yugi, and Mai went to save him and Kaiba. When Yugi and Seto returned after defeating the "Mythic Dragon", Adena revealed herself to be the "Mystical Elf", and she revived their fallen comrades Mai, Joey, Mokuba, and Earu.