Tenma Momoe

He is a student at the same school where Harutora, Touji, and Natsume attend. They are also his classmates.

He ended up unwitting involved in the Yakou Disciple incident when he was caught in attack from a shikigami called a Kodoku meant for Harutora. He however resolved himself to help Harutora rescue Natsume after listening his request. Since then Tenma has been drawn in various events related to the cult despite his somewhat cowardly and weak nature. He is quite knowledgeable on various info relating to current events of the Onmyodo world at large and sometimes helps Harutora catch up with his studies by lending him his notebook.

[Spoiler] It was later revealed that he is from a prestigious onymouji family. He is also the successor of the Momoe family and Witchcraft Corporation, his deceased parents' company who currently manufacture man-made shikigami for general purposes and making the use of shikigami accessible to all people. [/Spoiler]

(Source: Wikipedia)