Zenki and Goki

Zenki and Kouki (前鬼 & 後鬼, mae oni & nochi oni) are two Shikigamis, once caught by Asakura Hao in his first life, and later fell under the control of Kyōyama Anna

About
Zenki and Kouki are a pair of paper Shikigamis (old nature spirits who have become powerful demons) that were captured by Hao during his first life. They are very loyal to Asakura Hao and will kill anyone who comes in their way. Both have a striking similarity to ogres. However after being caught by Anna they seem to loose their conscience.

Zenki
Zenki is the blue Shikigami. He has a large horn in the middle of its forehead and two downward pointing tusks. It is large and rather muscular. He uses melee combat as his main form of fighting (Although in the manga he carries a shield as well). Its eye sockets are shaped as the "Seven Star Tree of Renewal"

Kouki
Kouki is the red Shikigami. He has two large horns on its forehead and two upward pointing tusks. Like Zenki, he is large and muscular and uses a large battle axe for combat. His eye sockets are shaped like the "Five Point Star". Unlike Zenki, he doesn't have eyes in each point of the star, instead he has one in the center.

History
They where first seen 1000 years ago during the time of Hao's first life. After becoming an adult, Hao was tasked with capturing the two ogres. After finding them he subdued them with ease and sealed them within two pieces of talisman's. Afterwards they became his loyal servants to Hao and later became guardians of and worked as a curse killing anyone who would break their seal on the book.

O.S. Zenki and Kouki
O.S. Zenki & Kouki(O.S.前鬼 & 後鬼)

Description: Once powerful shikigami sealed within a parchment seal by Hao to protect the Ultra Senji Ryakketsu. Now they obey Anna, who subdued these fearsome spirits and made them her Spirit Allies. With her mastery of the Ultra Senji Ryakketsu, Anna can easily invoke the two ogres as O.S. Zenki is formidable at melee-combat, using only his fists while Kouki wields a mighty battle axe. They are also big and very intimidating.
 * Shaman: Kyōyama Anna
 * O.S. Type: Spirit Type
 * Spirit Ally: Zenki & Kouki (Shikigamis)
 * Medium used: The 1080 Rosary Beads

Trivia

 * Zenki and Kouki are a reference to the Oni, which is the traditional representation of Demons and Ogres in the japanese folklore. They're often seen in the colors red (with two horns) and blue (with one horn) while using a wearing tiger-skin loincloths.