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. After Rukia Kuchiki was brought back to Soul Society, her Gigai was also brought along as part of the investigation into her disappearance. Hiyosu is at the Shinigami Research and Development Institute, watching footage of Rukia while in her Gigai. He comments on how the Gigai was well made, and chuckles to himself as he waits in anticipation for its delivery. He then is interrupted when Akon notes to Hiyosu that he is looking at the footage again. Akon and other researchers arrive as Akon comments on how he has been coming to the Institute everyday and wonders how Hiyosu still has not tired of looking at the footage of Rukia's Gigai. Hiyosu tells Akon to shut up as he was the one to find the Gigai, hence the Gigai belongs to him and how he looks at it does not concern Akon. Akon admits that the Institute was only able to find the unusual Gigai due to Hiyosu's close monitoring of the Living World. Hiyosu smiles and affirms this, adding that he would have already examined the Gigai had Akon not told him that some "secret organization" ordered him to come investigate. Akon tells him to not to say that and informs Hiyosu that he came today to bring the Gigai to him. A sound then alerts them to the arrival of the Gigai. Hiyosu then exclaims his eagerness and removes the cover from the Gigai. He happily expresses relief that the Gigai is undamaged, drooling over the chance to study it. Other fellow researchers make their way over to the Gigai, wishing to see the Gigai and be part of the study. One researcher examines the arm and finds that it is indeed well-made, but cannot find its maker's name anywhere. He asks who made it. Another researcher suggests that it might be Akon even though he does not seem to be the type to make Gigai like Rukia's. However, Akon replies that he does not remember ever creating such a Gigai. One of the other Shinigami Research and Development Institute members simply states that it does not matter who made it as long as he can do whatever he wants with it. Hiyosu takes a closer look at the Gigai, extending his left eye with the crank on his left temple as he tells the others to thank him since he found it. He is then shocked upon finding something with his examination and tells the others to come look. The others are perplexed and wonders if there is a problem. Akon also notices and is deeply surprised. Akon reveals that the Gigai was not made by anyone in the Institute or in Soul Society, and expresses his disbelief on how there shouldn't be anyone able to make such a Gigai. One of his colleagues replies that there may be someone despite Akon saying there shouldn't. Akon agrees, saying that while he does not know what is going on, they have have something very astonishing before them. He tells them all all that he can say that the person who made the Gigai is no longer in Soul Society. As his colleague asks why, Akon simply remarks that someone with such skill would surely have been exiled from Soul Society.

Bount arc
During the initial stages of the Bount Invasion, Hiyosu is seen with Rin Tsubokura assisting Captains Shunsui Kyōraku and Jūshirō Ukitake as the two captains investigate a strange phenomenon in the Living World. He and Rin are helping analyze the data that Suì-Fēng and Yoruichi Shihōin send in from the Living World. When Suì-Fēng and Yoruichi Shihōin find the remains of one of the human victims, they send Soul Society traces of the reishi near the victim to be analyzed. As the data comes in, Hiyosu is surprised to see that the data cannot be identified. He scolds Rin, thinking that the analysis was done incorrectly and orders Rin to try again. As Yoruichi and Suì-Fēng continue in their investigation, they come upon a Bount about to devour the soul of a human train conductor. Video information of this encounter is sent to Hiyosu and others as Yoruichi stops the Bount. The captains are surprised at what they see. With the data they have, Ukitake has Sentarō Kotsubaki lift the temporary closing of the Senkaimon that was done as part of the survey as Ukitake and Kyōraku ends their survey investigation for the night.

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.