(小山田まん太,Oyamada Manta, Mortimer "Morty" in the English anime and video games)
Manta is an intelligent child who carries a book around with him every day. In the Japanese versions, the book is called the Manjien (万辞苑), meaning "Manta's Book on Everything", while the English manga calls the book the "Mantannian Dictionary". "Manjien" comes from Manta's name and the Japanese dictionary "Kōjien" (広辞苑). Manta is 13 years old and a 7th grader at Shinra Private Academy; Manta goes to school with Yoh and Anna.
In the English anime, Morty is a fan of ghost stories and the book he carries around is a ghost story book.
Manta can see spirits, but cannot use their powers like his best friends Yoh Asakura and Anna Kyōyama can. He is very expressive and a bit panicky. He physically resembles a Koropokkur (known as "minutians" in the English anime), and has been mistaken for one of the shy nature spirits on several occasions
In the anime, during the Shaman Tournament, Manta became a shaman, his spirit is Mosuke, Amidamaru's best friend who forged Harusame (Sword of Light in the English anime). He is however not capable of maintaining spirit form because he runs out of furyoku and ends up in trouble. In the battle with Hao, he used his oversoul to knockout Hao's Shikigami's. His ghost combines with his laptop, where he is usually at and cannot escape, and forms a hammer. This can also be seen as a parallel to the relationship of Amidamaru and Mosuke as neither Amidamaru and Mosuke nor Yoh and Manta were away from each other for very long and Mosuke and Amidamaru are spirit partners with Manta and Yoh respectively.
In the manga and original Japanese anime, his family owns the Oyamada Electronics Company, and therefore is very rich. Manta's parents and little sister only appear in the manga. Manta's father, Mansumi Oyamada, is 55 years old. His mother, Keiko Oyamada, is 37 years old. His sister, Mannoko Oyamada, is 5 years old. Manta's family plays little role in the story, and, following their initial appearance, they are never seen again in the manga until a brief appearance by Mansumi toward the end of the series, where the Oyamada president leads several battle ships to destroy and capture Shamans.
In the manga and Japanese anime, Manta has an assistant, Tamurazaki. In the English anime, Tamurazaki (who turns out to be Mansumi's lapdog and guns down at least one of Hao's followers) is "Uncle Jerry" and is Morty's uncle.
Manta's name in the Fourkids dub was 'Morty' or 'Mortimer'.
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