Hiyosu

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

Appearance
Hiyosu is a large, pale green (dark green in the anime) 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 had a more orthodox complexion, and 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 Ishida'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 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. Commenting on how the Gigai was well made, he chuckles to himself as he waits in anticipation for its delivery. He is interrupted when Akon notes 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. When Akon admits the Institute was only able to find the unusual Gigai due to Hiyosu's close monitoring of the Living World, Hiyosu, smiling, affirms this, adding he would have already examined the Gigai had Akon not told him some "secret organization" ordered him to come investigate. Telling him to not to say that, Akon informs Hiyosu he came today to bring the Gigai to him.

As a sound alerts them to the arrival of the Gigai, Hiyosu, expressing his eagerness, removes the cover from the Gigai. Happily expressing relief the Gigai is undamaged, he drools over the chance to study it. Other researchers, making their way over to the Gigai, wish to see the Gigai and be part of the study. One researcher, examining the arm, finds it is indeed well-made, but, unable to find its maker's name anywhere, asks who made it. Niko Kuna suggests it might be Akon, who states he does not remember ever creating such a Gigai. As Hiyosu, taking a closer look at the Gigai, extends his left eye with the crank on his left temple, he tells the others to thank him, since he found it. Shocked upon finding something with his examination, he tells the others to come look. Akon reveals the Gigai was not made by anyone in the Institute or in Soul Society.

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

Arrancar arc
During Yammy Llargo and Ulquiorra Cifer’s first arrival in Karakura Town to gather information on Ichigo Kurosaki, Hiyosu is present at the Research Institute. Along with other members of the Institute, he is keeping watch on any unusual spiritual readings in the Living World. As they initially do not detect anything, one of Hiyosu's colleagues complains on how there is nothing to be detected, and how it is really boring to keep watch when nothing occurs. Scolding Rin for eating, he 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, giving them the coordinates, asks for confirmation. Hiyosu and the others get to work immediately. As Akon walks in on them, Hiyosu, stating Akon came at the perfect time, tells him to come over and check their readings, as the Arrancar have made their appearance. Hiyosu and the others make haste to send Captain-Commander Genryūsai Shigekuni Yamamoto a message of their detection of Arrancar activity, and the Arrancar are likely “completed.”

Hiyosu and the other members of the Research Institute are hard at work again during the third invasion by the Arrancar at Karakura Town. Rin informs them of the detection of the Arrancar. As they monitor the situation, Hiyosu, checking on the limiter status on Captain Hitsugaya's advance guard, is informed they have been granted permission to release their limiters. When he asks Rin what the level of the detection is, Rin informs them all it is “Espada” level, shocking Hitosu and his colleagues. 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.

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

The Thousand-Year Blood War arc
As Akon argues with Kisuke Urahara, Hiyosu, listening, notes the only people who can make him lose his cool are the captain and "that person". Hiyosu tells Rin Tsubokura to begin circulating word 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.

Much later, as the Seireitei is being consumed due to the Quincy's invasion, Hiyosu works with the rest of the SRDI to try and figure out what has just happened to the Seireitei. In complete disbelief over the readings his instruments were giving him Hiyosu quickly becomes panicked once the building they are in is consumed by shadows. Appearing behind them, Askin Nakk Le Vaar explains the nature of the 'shadows', and reveals that by creating space within the shadows, the Wandenreich had always been in the Seireitei.

Ten years after Yhwach's defeat, Akon wonders where Mayuri is, concluding that he is attending an event for the new 13th Division captain. Hiyosu tells him that he believes Mayuri has in fact gone off to investigate the reconstructed sectors. Angered by this, Akon asks why he would do that, but Hiyosu replies that since Akon, who is the lieutenant, does not know, then he could not know either and asks what has happened. Akon informs him that he has noticed readings on the 12th Division's sensors nearly identical to those emitted by Yhwach ten years prior.

Powers & Abilities
Kidō Practitioner: Hiyosu is capable enough with Kidō to create barriers to seal off an area of the SRDI when their building came under attack.

Corporal Modifications: Hiyosu has subjected his body to some bodily enhancements, specifically in his left eye which can be extended in such a way that it comes out of its socket, to do this Hiyosu turns a crank which has been implanted in his left temple.

Zanpakutō

 * Shikai: Not Yet Revealed
 * Bankai: Not Yet Achieved

Trivia

 * He is one of the few Shinigami within the Gotei 13 who does not take on the appearance of a Human.
 * 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 in the Human World.