Shikigami(式神) are a kind of spirit summoned to serve a practitioner of onmyōdō, much like a western familiar. Shikigami cannot be seen by most people, but according to the Heian period onmyōji who were said to control them, shikigami often looked something like a child-sized oni demon. Although invisible, shikigami supposedly could, at the onmyōji's command, take a variety of human or animal forms, possess or bewitch people, and even cause bodily harm or death.
The famous onmyōji Abe no Seimei was believed to be particularly skilled at manipulating shikigami, and even had them performing odd jobs around the house. The Twelve Heavenly Generals were said to be among the shikigami that he employed.
The 12 summons have been in existence for a very, very long time and can be considered as gods of the lower rank. Currently they fall under the command of Abe no Seimei, the greatest Onmyouji of all times. Unless Seimei summons them, they usually reside in the spirit world of the gods.
These 12 summons are special as they were actually created by human dream's. Overtime, people's dreams and imaginations were transformed into the 12, giving them unique forms and personalities. They can be considered as the children of humans. Hence, the golden rule of the 12 is that they must never harm a human let alone kill one. Should they break that rule, they will lose their souls & become demons.
Although they are gods possessing strong powers, the summons can get injured, can feel pain and can lose their lives. When one of them dies they will be reborn instantly as people's dreams never stop giving them form but they will forget their past self and continue on as a completely different god.
Guren (紅蓮, Guren?)/Touda (騰蛇, Tōda?)/Mokkun (もっくん, Mokkun?)
Touda is one of twelve Shikigami in service to Seimei; who gave him the name Guren after Touda pledged his servitude. The form he takes most of the time is a cat/dog like mononoke whom Masahiro named Mokkun. Because he is invisible to anyone without a spirit sense, Mokkun frequently plays tricks on people who annoy him, much to Masahiro's chagrin. From the time Masahiro was a baby, Guren had 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 Touda share a deep sense of friendship and trust between them and both are willing to lay down their own lives for the other. In his past, a spell was cast upon Guren by Seimei's best friend Ryuusai who ordered Guren to kill Seimei and after that Guren kills Ryuusai. Also because of that incident Seimei's other Shikigami don't trust him. It is also mentioned that Guren is one of most feared summon Gods, possessing a power so great (controlling the fires of Hell) that Seimei created a golden ring to wear around Guren's head so as 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 worthy to be his successor. In the form of Mokkun, he 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.
Later in the series, he is controlled by Chishiki no Guuji and fatally wounds Masahiro. At the end of the anime, Masahiro chooses to kill Guren as opposed to leaving his soul corrupt. He later revives him in exchange for his own life and for Guren's memories to be sealed away so he would not have to remember the terrible things he has done. Despite Guren having no memory of who he is, Masahiro felt this was better than seeing him die and becoming someone else. Fortunately, before Masahiro crossed the river to the other world, a woman whom revealed herself as his grandmother, Wakane. She showed him that it's too early to cross it so she returned him to life and went well.
Kouchin (勾陣, Kouchin?)
Kouchin appears as a mature-looking woman, about 25, with shoulder-length black hair. She is ranked second strongest of the twelve in attack power, after Touda. Kouchin specializes in close range fighting with twin forks. Unlike the others, Kouchin does not resent Touda as much and is actually his close friend (and possibly lover) as Touda refers to Kouchin as 'Kou'. When Masahiro asks him about this, he refused to respond. She has also watched over Masahiro when he was a baby together with Guren. She is very observant and has a clear grasp of all of the 12's personalities. She is also very good at noticing small details. Her second name from Seimei is Keito (慧斗).
Rikugou/Saiki (六合, Rikugō?)
Rikugou's primary role is to be the on-call bodyguard to Seimei but he will leave Seimei's side to aide Masahiro should he be ordered to. Rikugou's sense of loyalty is so strong that he was willing to put aside his doubts regarding Masahiro's abilities and accepted him as Seimei's successor on little more than his master's assurances. He differs from the other Sheishou in that he rarely use magical attacks, preferring instead to fight opponents head on with a barbed spear. Rikugou can also use his cape as a shield and create an explosive 360 degree shock wave of air by striking the ground with his weapon. He also has a chain that was used against a 'controlled' Guren but Guren overpowered him. Rikugou is a man of few words and only speaks when he must. The only way to notice any change in his emotions was through his eyes. This began to change, however, after he met Kazane whom he fell in love with. Rikugou saved Kazane's life two times. Once when she was swallowed by a demon and the other is when they were in the underworld. He began to show worry, frustration and was furious when she was killed. Just before she died, they shared an emotional embrace and he revealed to her that his name from Seimei is Saiki(彩輝), and that she is the only one he told his name to. He also gets along well with Mokkun.
Seiryu (青龍, Seiryu?)
Seiryu is stubborn, antisocial and has a bad temper. He holds a deep loathing for Guren due his attempted murder of Seimei and also considers Masahiro to be unworthy to be Seimei's successor. Seiryu even went as far as to say that if Guren ever harms a human again, he will personally kill him. He attacks with blasts of lightning and fights with a large two handed scythe. His second name from Seimei is Shouran (宵藍). Seiryuu cares about Seimei's wellbeing more than any of the others. However he always shows his care through anger.
Suzaku (朱雀, Suzaku?)
Suzaku is a fire user like Touda but unlike Touda's destructive flames, Suzaku's flames are used to purify evil spirits. He uses a heavybladed Zanbato to slash his enemies. Suzaku has bright red hair and wears a white strip of cloth around his forehead, which was Tenitsu's. He is very much in love with Tenitsu and is fiercely protective of her due to her wound transfer ability, going as far as to hit Masahiro after Tenitsu saved him. The reason he cares about her was because Tenitsu past self was Suzaku's best friend who was reborn right before his eyes after she died. Not wanting to lose her again, he becomes her protector. His sword which has the power to kill gods was later given to Masahiro to use against Touda.
Tenitsu (天一, Ten'itsu?)
A member of Seimei's Shikigami, Tenitsu has the appearance of a young woman with long blond hair. Her ear-ring is Suzaku's. Unlike the other members of the Sheishou, she specializes more in magic than combat, particularly healing and clairvoyance. When someone has a very serious wound, she heals it by transferring the wound onto herself, something her lover and fellow Shikigami Suzaku distresses over. Tenitsu has another name, Tenki (天貴), which only Suzaku calls her by. She is usually seen protecting Akiko or just keeping her company. She is the only Shikigami of the 12 that has died before. Her past self wore her hair short and was loud and energetic. She was also the best friend of Suzaku.
Taiin (太陰, Taiin?)
Taiin is a wind user who resembles a young girl with high pigtails. Fittingly she also has a childish personality despite living for so long. Said to be rash by a fair few of the Seishou, Taiin attacks with mini tornadoes and can also use the wind to communicate and pass messages. She can also 'read' the wind to know what's happening at other places but she's not good at it. Because of her personality she is often at the receiving end of one of Byakko's lectures with the current record lasting 8 hours. Taiin does not like slimy things and often appears with Genbu. She can create a tornado to transport people to other places but the ride is usually very rough.
Genbu (玄武, Genbu?)
Genbu resembles a young boy of about ten years. Although not possessing much ability in combat, Genbu specializes in defense and making barriers. However, out of the four people that can make barriers his is the weakest. He is a water user and has a water mirror that allows the user to see things happening far away. He can also use this mirror to communicate with fellow water user Tenkou. Genbu is often seen with Taiin who he constantly berates on some of the things she does.
Tenkou (天后, Tenkou?)
A water user like Genbu. Tenkou appears to be about the same age as Kazane and has long, light sliver hair. She gets along well with Kouchin. Tenkou is the weakest out of the eight fighting summons and is angry at herself for that. She is about as stubborn as Seiryuu but has her gentle side. She usually apologises on behalf on Taijou when he's made Seiryuu mad.
Taijou (太裳, Taijou?)
A man with green hair and lavendar eyes. He appears as a very peaceful, respectful man who always call Seimei's family with the suffix "sama". He used to take care of Narichika when he was young and was the one who taught Masachika how to use the bow and arrow. Taijou usually resides in the spirit world along with Tenkuu. He has the second strongest barrier making ability. Although being polite to everyone, Taijou seems to not think when he talks, especially to Seiryuu, and never realises that he's made him angry until it's too late.
Byakko (白虎, Byakko?)
Byakko resembles a strong middle-aged man with grayish-green hair. Like Taiin, he is a wind user but his wind is more 'gentle' and travels slower than Taiin's. He treats Taiin and Genbu like his children. He is also the only one that can keep Taiin under control.
Tenkuu (天空, Tenkuu?)
A summon that resembles an old man with a long beard. Tenkuu is the governor of the 12 and rarely leaves the spirit world. He also has the strongest barrier making and defense powers of the 12. So strong, in fact that he can immobilize enemies without leaving the spirit world. He also makes all the Shikigami's weapons.