Tosaka

is a researcher who is affiliated to the Land of Rivers.

Personality
Originally, Tosaka was portrayed as a very eccentric man, who shows great passion and effort in his work, even climbing up a tree to return a nest with its young. He feels the best way to understand something is to apply all five senses to it, even going as far as to lick the crudest of things.

Later, however, this nature was revealed to be a façade. He is in reality a shameless sociopath with little regard to life, taking sadistic pleasure in watching others struggle. Similar to the non-reformed Orochimaru, Tosaka's one goal is research and scientific growth beyond all else, he is willing to do so through amoral means, even inhuman testing. At the same time, he views his own intellect and technology as far ahead of the Sannin. His obsession with his research has led him to have no loyalty towards anyone, delightfully manipulating all around him, even killing them, to accomplish his goals. His unhinged nature even is shown to look past his own failings to instead watch with joy other people's struggles.

Appearance
Tosaka is a fairly tall man of light complexion and pale-green eyes. He has silver hair that is chin-length with short bangs over his eyes. He wears a beige bodysuit with a long orange shirt over it and brown gloves along with brown sandals.

Abilities
While apparently not a shinobi, he was requested to join a ninja team on an investigative mission due to his extensive bird knowledge. He is also an adept scientist in other fields, including chemistry and biology, having been trusted in heading the research and development of cursed seals. Even more, he is apparently an adept engineer, having designed Scientific Ninja Tools that simulate cursed seals and augmenting their power. Most noticeably, he is proven to be very tactical and deceptive, having planned and manipulated all around him to further his research.

Jūgo Arc
When word reached Konohagakure about birds randomly attacking a village, Tosaka was requested to work alongside Team 7 and Team 15. Along the way, Tosaka would continuously analyse any evidence of birds in the area, at times grossing out some of his allies. Upon arriving, the group was attacked by strange-looking birds. At the village, they learned that many people attacked by the mutated birds quickly became sick and were covered in Cursed Seals. The villagers also found an unconscious Jūgo, who was recognised by Sarada and Mitsuki. When the man awoke and was questioned, one the sick villagers went on a feral rampage and attacked Tosaka until he was suddenly drawn to Jūgo.

Tosaka aided Konohamaru moving around after his fight with Jūgo. They followed Team 7 until they found a passed out Jūgo with an injection device by his hand. Tosaka examined him to compare his condition with the birds' affliction, and commented on finding the natural habitat of an endangered bird species. Jūgo woke up just in time to protect them from another bird attack, curing it of its cursed seal before leaving. Back at the village, he corrected Wasabi when she brought up the possibility of Konohamaru contracting Jūgo's cursed seal through his injuries, his analysis from Jūgo's blood sample revealing his cursed seal and the birds' were different. When Mitsuki confirmed the device they found was from Orochimaru's research, Tosaka added it was a powerful tranquilliser, having licked it. Konohamaru tasked Tosaka with helping Team 7 monitor Jūgo and look out for more of the tranquilliser. Tosaka was touched by Jūgo's effort to cure the birds despite the strain it caused him. After Jūgo relented in letting them help, and explained how to determine if a bird was infected even before they turned violent, Tosaka suggested that finding the source of the very territorial birds could help them figure out where the cursed seal came from. Later, Tosaka and the others discussed the lack of progress in their investigation, and wondered if something else was happening, while being spied on by one of the Land of Rivers researchers Team 7 met earlier.

Over the next couple days, Tosaka aided the group in curing more birds. Sarada explained to him she convinced Jūgo to take his shots regularly instead of only when he feels a transformation coming. Tosaka identified a domesticated goose among the wild ones, and commented to Boruto the species lost the ability to fly. On the third day, Tosaka claimed the still infected birds transformed and broke free, with Jūgo going after them. Jūgo attacked them as he underwent a complete transformation, his spare injections having disappeared. Jūgo began attacking the village, but timely intervention from Land of Rivers researchers knocked him out with tranquillisers before he could hurt anyone. After exposing the Konoha shinobi as being aware of Jūgo being the monster, the researchers turned the village against them. When they mentioned having requested backup, the researchers showed them Wasabi and Namida, unconscious and infected with cursed seals, and requested Tosaka's help due to his expertise. Tosaka promised Boruto that he will help Jūgo and the birds.

As he began working on Jūgo, he was approached by Boruto and Suigetsu Hōzuki. They explained that the Land of Rivers researchers were behind the cursed seals outbreak and were after Jūgo to harvest his genetic material as the original cursed mark. As he brought the two to Jūgo, Tosaka incapacitated them with tranquillisers, revealing that he was actually in a league with the researchers. He also revealed that he started the outbreak to lure Jūgo out in the hopes of weaponising his cursed seal. He injected Jūgo with a serum to force his transformation. When the research team came and tried to stop him, Tosaka revealed his already-finished cursed seal weapon, transforming to kill his cohorts. As Jūgo continued to transform, Tosaka was delighted to test his weapon against an original cursed seal's power.

As Jūgo escaped and returned to the lake with the infected birds, Tosaka followed close behind. Boruto attempted to stop Tosaka, but the crazed scientist his weapon's second form to easily subdue Boruto. Once Jūgo fully transformed, Tosaka unleashed his weapon's power on the Oto-nin, who effortlessly defeated Tosaka, much to Tosaka's horror. As Jūgo continued to go berserk, the crazed scientist laughed at Jūgo's harsh destiny. Boruto however refused to believe that this was the only path for Jūgo. In a desperate gambit, Boruto launched himself with his was able to hurt Jūgo enough to knock him out of his rampage, shocking Tosaka. After the group was able to cure the birds, Jūgo's team slipped away with Tosaka as their hostage, much to his anger.