Otoha

Otoha appears to be a normal girl, but she is in fact a ghost, and only Takuma can hear or see her. She is always cheerful, and will appear out of no where, clinging suddenly to Takuma. She refers to herself with the masculine pronoun boku, meaning "I". She refers to Takuma as the "Promised Person".

[Spoiler] It is later known in the anime that she is the actual Kagura Hinata. [/Spoiler]

(Source: Wikipedia)