Zenki and Goki

Zenki and Kouki (前鬼 & 後鬼, Zenki & Goki) are two Shikigami used by Asakura Hao in his first life, and later fell under the control of Kyōyama Anna

About
Zenki and Goki are a pair of paper Shikigami that were captured and used by Hamo Tadatomo and later came into the possession of Asakura Hao during his first life. They are very loyal to Hao and will kill anyone who comes in their way. After being caught by Anna they seem to lose their conscience.

After Hao's first defeat by the Asakura Family they are sealed within the Chō-Senjiryakketsu from where they will try to kill anyone who would attempt to learn the content of the book. During the Shaman Fights in the year 2000 Tamamura Tamao accidently releases Zenki and Goki from the book where they are quickly defeated by Kyōyama Anna who reseals them within her 1080 Prayer Beads, making them her own.

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 and in the manga he carries a shield as well. Its eye sockets are shaped as the "Seven Star Tree of Renewal"

Goki
Goki 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.

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

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

Anime/Manga Difference
In the anime it is Hao who captures the two ogres. A flashback is shown where Hao is seen confronting and sealing them within two paper talismen.

Trivia

 * Zenki and Goki 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 wearing tiger-skin loincloths.