Guren is one of twelve Shinshō in service to Seimei, who gave him his current name, after he pledged his servitude. His original name is Tōda, (騰蛇). However, the other Shinshō still address him by his original name. Only Seimei and Masahiro call him Guren.

The form he takes most of the time is an ayakashi although Masahiro considers him a mononoke, whom Masahiro named Mokkun. Because he's invisible to anyone without a spirit sense, Mokkun frequently plays tricks on people who annoy him, much to Masahiro's chagrin. It was also shown in the anime that Guren dislikes babies, because according to him, babies often cry for no reason. However, Masahiro disagrees, saying that Guren was merely afraid that the children who saw him would hate him.

From the time Masahiro was a baby, Guren watched over and protected him, as Masahiro was the only person who wasn't afraid of him. Because of this, despite how they constantly bicker with each other, Masahiro and Guren share a deep sense of friendship and trust between them and both are willing to lay down their own lives for the other.

It is also mentioned that Guren is one of most feared summon gods, possessing the power of controlling the fires of Hell. Seimei also created a golden ring to wear around Guren's head to keep his own strength from overwhelming him.

Since Masahiro doesn't fear Guren, Seimei cites this as the proof his grandson is the only one compatible to be his successor. In the form of Mokkun, the familiar is able to erect barriers and project crescent shaped blades of energy; in the form of Guren, he attacks with blasts of fire and a trident.

In the past, Seimei’s best friend Ryūsai who ordered Guren by using the Baku-kon spell, a spell that could forcefully control anyone’s heart. He used it on Guren to kill Seimei and after that kill him. In addition, because of that incident the other Shinshō don't trust him. Nevertheless, that began to change with Masahiro's help, as stated by Kōchin.

Later in the series, he is corrupted by Chishiki no Gūji, losing his soul and becoming a cold-blooded monster who almost leaves Masahiro to die. It is later revealed that he’s a slave of the Underworld, as he controls the Fires of Hell. His blood was required to break the final seal of the gate to the Underworld.

At the end of the anime, Masahiro revives Guren; saving his life in exchange for Guren's memories of Masahiro being sealed away, so he would not have to remember and regret the terrible things he has done. Despite Guren having no memory of who he is, Masahiro felt that this was better than seeing him die. However, Guren is sill one of Seimei's familiars. As such, he is under orders from Seimei to continue to look after Masahiro.

(Source: Wikipedia)