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Kokuyou (黒曜 Kokuyou?) formerly known as The Black Monarch (玄帝 Gentei?) is a companion and contracted beast of Touya on his journey. He is the half of the Genbu (玄武 Genbu?), the other half being Sango, and they are one of the Four Heavenly Beasts and also the leader of the Scaled Creatures.
Kokuyou's true form is a large jet black serpent as big as an anaconda with a pair of golden eyes. Kokuyou and Sango are a union being and inextricable. After Kohaku told them about the trouble that they might cost because of their form, they then changed their appearance in similar way to the Kohaku's form.
The character introductions state that Kokuyou is "male, but female at heart". He is shown to be effeminate, as he speaks in a feminine way and uses feminine pronouns. This is despite him being the only male member of the Four Heavenly Beasts.
The Genbu (玄武 Genbu?), which later known as Kokuyou and Sango, are one of the divine beast that can manifest in the world once every few decades or centuries, unless they are being summoned by human .
- Intimidation Aura: Kokuyou and Sango are able to release suppressing aura that render all individual, which have lower magic level in respect to the divine beast, becomes immobile.
- Long-Ranged Telepathic Communication: Kokuyou and Sango are able to make telepathic communication with their master and among the divine beasts regardless the distance. They also able to include other people into this communication line.
- Form Transformation: Kokuyou and Sango are able to change her menacing true form appearance into cute young beast form (Child Form) and back as they pleased.
- Magic Barrier: Kokuyou and Sango have magic barrier that is able to repel all type of physical attack completely and also all type of magical attack with respect of the caster magic level.
"That is so. Please choose a lovely name, Goshujin-sama." - Kokuyou answering his new master, Touya.
- According to the character introduction, even though Kokuyou is a male, in heart he is a female.
- His name, Kokuyou (黒曜), is a Japanese word for the gemstone Obsidian.
- Their epithet, Gentei (玄帝), is composed by "gen" (玄) of Genbu and "tei" (帝) which means sovereign, emperor, or monarch in general. Literal translation of this word is "monarch of Gen", which is meaningless. However the character "gen" (玄) of Genbu can be also read as kuro (くろ kuro) which means black. Therefore, Gentei (玄帝) is best translated as black monarch.