Hiyosu

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

Appearance
Hiyosu is 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. 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.

Previously, his skin was a more orthodox complexion, his head was wider with a clear border between the head and neck.

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
Shortly before her arrest, Hiyosu discovers Rukia Kuchiki hiding in the Human World using an unusual Gigai. When Rukia is brought back to Soul Society, her Gigai is 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 that he is looking at the footage again. Akon and other researchers arrive as Akon comments on how he has been coming there a lot to look at Rukia's Gigai. Hiyosu tells Akon to shut up as since he found the Gigai it 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, but he replies that he does not remember ever creating such a Gigai. 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. Akon also notices and reveals that the Gigai was not made by anyone in the Institute or in 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
During Yammy Llargo and Ulquiorra Cifer’s first arrival in Karakura Town to gather information on Ichigo Kurosaki, he is present at the Research Institute. He along with other members of the Institutue are keeping watch on any unusual spiritual readings in the Living World. They initially do not detect anything. One of Hiyosu’s colleagues complains on how there is nothing detected and how it is really boring to keep watch when nothing occurs. Hiyosu scolds Rin for eating and orders him to go get them some tea. As Rin apologizes, he notices the detection of Yammy’s and Ulquiorra’s reiatsu. The researchers are alerted as Rin gives them the coordinates and asks for confirmation. Hiyosu and the others get to work immediately. As Akon walks in on them, Hiyosu states Akon came at the perfect time. He tells Akon to come over and check their readings as the Arrancar have made their appearance. Hiyosu and the others then make haste to relay Captain-Commander Yamamoto a message of their detection of Arrancar activity and that the Arrancar are likely “completed.”

Hiyosu and the other members of the Research Institute are seen to be at hard work again during the third incursion of the Arrancar at Karakura Town. Rin informs them of the detection of the Arrancar. As they monitor the situation, Hiyosu checks on the limiter status on Captain Hitsugaya’s advance guard. He is informed that they have been granted permission to release their limiters. He then asks Rin what is the level of the detection. Rin informs them all that it is “Espada” level, causing Hiyosu and his colleagues to have looks of shock. The other members of the Gotei 13 in Soul Society are informed of the situation.

Beast Swords arc (anime only)
Note: Events occurring in this arc are only in the anime and do not constitute canon material. 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 between Soul Society and World of the Living, Hiyosu receives a message from Rukia Kuchiki inquiring about the possible Gigai they found. Akon instructs him to ignore it, but he insists that Akon look at it. Kagerōza Inaba then comes in and Akon instructs Inaba to lead Captain Kuchiki and the others as they prepare to enter the Dangai to do a survey. Inaba complies. Hiyosu then informs them that they have established contact with Captain Kurotsuchi. Kurotsuchi wonders why they took so long. Akon apologies as he had trouble adjusting the communications. Akon asks on the situation, which Kurotsuchi replies that the cause is man-made as he suspected. Kurotsuchi then asks if Captain Kuchiki and the others are ready to enter the Dangai. Akon informs his captain that they are about to enter. The Institute then loses contact with Kurotsuchi. As the situation starts to look much more serious than what it was initially thought to be, Hiyosu is at the Institute examining some data. He hears a knock at his door and promptly answers, asking who it is. He looks around the hallway before he is knocked backwards by a Kidō spell from Rukia Kuchiki.

The Thousand-Year Blood War arc
Hiyosu listens as Akon argues with Kisuke Urahara, noting that the only people who can make him lose his cool are the captain and "that person". Hiyosu tells Rin Tsubokura to begin circulating word that Ichigo is on his way to Soul Society. When the SRDI is attacked by Jidanbō Ikkanzaka, Hiyosu tells Akon and the others to retreat as he tries to seal off the area. However, he is stabbed by Rin.

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.