Manta Oyamada is an intelligent but worrisome teen who always carries a "dictionary/encyclopedia/compendium of all human knowledge." He has a sixth sense, meaning he can see supernatural beings, even though he is not a Shaman. He is very expressive and a bit panicky. His rather abnormal height has gotten him into trouble numerous times, once having been mistaken for one of the shy nature spirits, and being cut open and used as a hostage of sorts during You Asakura's match with Faust VIII.
Manta Oyamada narrates the story in the manga and anime, allowing readers and watchers to understand the plot more efficiently. He is referred to as "Morty" (short for Mortimer) in the English anime. (Hiroyuki Takei mentioned Manta's character was modeled after Japanese pop singer T.M.Revolution in an interview published in the March 2004 English language issue of Shounen Jump.)
(Source: Wikipedia, edited)
