Ittosai Kunieda

Aoi and Kouta's grandfather. He is the master of the shingetsu style and Kunieda's instructor. He has a violent personality and is a very stern man with a hair-trigger temper. He also adopts an attitude of fighting first, asking questions later.

[Spoiler] Not only does he know about demons and the demon world, but he also recognizes Oga as a contractor and decides to train him to use demonic powers. [/Spoiler]

(Source: Wikia)