Hiyosu

is a department head in the Shinigami Research and Development Institute, which is tied to the 12th Division.

Appearance
Hiyosu is first shown as a large, green skinned Shinigami with a bulbous head and a cleft across his scalp from his forehead to the back of his neck, marking his two hemispheres. His left eye is extendable via a crank in his left temple.

In a flashback from episode 295 his appearance was slightly altered: his skin was shown to be in a more orthodox complexion; his head was wider with a clear border between the head and neck.

On both sides of his neck, under his lower jaw, he has gill-like depressions. The cleft in his head features the marks of sewing, suggesting it is a scar from an incision.

History
He, alongside his captain Mayuri Kurotsuchi, Akon and two other Shinigami, experimented on Quincy, including Sōken Ishida, Uryū's master and grandfather.

Agent of the Shinigami arc
He was the one that originally discovered Rukia Kuchiki's unusual Gigai.

Bount arc
He is shown scolding Rin for scanning incorrectly when Suì-Fēng and Yoruichi Shihōin are investigating the Bounts.

Arrancar arc
He was present when the Research Institute discovered that Yammy Llargo and Ulquiorra Cifer had begun a small invasion on Karakura Town.

Beast Swords arc
Hiyosu, along with the other members of the Research Institute, has also been known to perform field research when requested by his captain, Mayuri Kurotsuchi, as shown during the Beast Swords arc.

Gotei 13 Invading Army arc (anime only)
Note: Events occurring in this arc are only in the anime and do not constitute canon material.

While a study is being conducted to identify the cause of a time gap in the Dangai, Hiyosu receives a message from Rukia. Akon instructs him to ignore it, but he insists that Akon look at it. Contact is established with Mayuri in the Dangai, who explains that the cause is man-made. Hiyosu hears a knock at his door and promptly answers, asking who it is, he is then knocked backwards by a Kidō spell from Rukia Kuchiki.

Trivia

 * He is one of the few Shinigami within the Gotei 13 who doesn't take on the appearance of a Human.


 * It is interesting to note that, in the anime, Hiyosu appears to alter the color of his skin from green to white between the invasion of Ulquiorra and Yammy and the briefing of Rin Tsubokura prior to his assignment on Earth.