In Mutant Ninja Turtles Gaiden, Sadao (サダオ) is a Foot Ninja directly under Karai's command. His comrade and male lover Ryōta Gotoh was killed by Leonardo Hamato, whose dark side went on a rampage under the Curse of Yamata-no-Orochi. But over years, Sadao forgave Leonardo and they became each other's foil as dear friends and comrades.
Sadao wears the standard uniform of a Japanese Foot Ninja, with standard armor and mask. As such, his outward appearance is meant to be identical to nearly every other Foot Ninja under Karai's command, with the exception of his friend and comrade Leonardo Hamato. In particular, Sadao is never seen unmasked, and he routinely declines Leo's requests to see his face.
In a lengthy flashback sequence in Chapter 21: The Curse of "Yamata-no-Orochi", Karai has just defeated Leonardo Hamato, who had gone on a rampage fueled by his Curse of Yamata-no-Orochi, indiscriminately killing Foot Ninja. Karai has taken Leonardo to a nearby Foot Clan facility where the dead and wounded ninja have been gathered. As Karai guides Leo through the facility, a certain ninja is seen mourning over one of the corpses.
Not long after, Leo has unwittingly identified fugitive former Foot Ninja Douan Morinaga as a local kendo instructor, and Karai immediately orders his assassination as a descendant of the Morinaga Bloodline. Even as Leo argues with Karai over the justifications of killing people, Karai addresses the mourning ninja as Sadao, and orders him to attend a meeting. Sadao instead approaches Leonardo and tries to attack him. Leo insists he comes in peace, but Sadao points out that his late friend Ryōta was one of the two ninja who originally approached the turtle in peace, and yet he killed him. Karai orders Sadao to contain his emotions and follow orders, promising him he can mourn later.
After Leo tries and fails to save Douan from a now-imminent death, Sadao confronts him alone, vowing to avenge Ryōta. Leo apologizes for killing Sadao's comrade, repeating the explanation that he was under the effects of the curse. But Sadao explains that Ryōta wasn't just his comrade—they were lovers—and lunges forward to attack the turtle. Leo shifts into combat mode and defends himself, but his own dark side is again provoked in the process, and takes control of his body. Leo almost kills Sadao, but his good side immediately regains control the very moment Douan's life ends. Leo spares Sadao's life and immediately apologizes before returning to Karai.
In another flashback sequence in Chapter 22: The Witch-Hunt of a Bloodline, Karai has made Leonardo her second-in-command of the Foot Clan, and Sadao now reports to him. But Sadao resents the "kappa"—not just for killing Ryōta, but for his hypocrisy: Leo uses the Yata-no-Kagami to identify Morinaga targets and orders Foot Ninja (including Sadao) to eliminate them, but Leo himself refuses to personally kill innocent people. Sadao points out to Leo how much of a toll this mission of extermination is taking on the souls of his fellow Foot Ninja, noting that many have since committed suicide. Leo reminds Sadao that the curse has also unleashed his own dark side and killed many innocent people. But Sadao refuses to spare Leo any sympathy, instead directly insulting his Hamato Clan honor. This provokes Dark Leo into taking control and attacking Sadao, which appears to be exactly what Sadao wanted: Sadao can either kill Leo, avenge Ryōta and claim self-defense for it, or he can die trying and be reunited with Ryōta. Leo gains the upper hand and leaves Sadao at the edge of defeat. Dark Leo brags that his Hamato Clan honor is perfect, and that he can do anything he needs to do, including killing his own father Splinter to free him from the Curse of Yamata-no-Orochi. Sadao is galled by Leo taking such pride in euthanizing his father in comparison to the Foot Ninja killing thousands of mostly innocent people who could not fight back. Sadao mocks Leo for only killing easy Morinaga targets who were already evil, and suggests Leo is too weak to live with any guilt worse than killing his father. Dark Leo angrily impales Sadao with his katana. But Good Leo acknowledges that Sadao was correct about everything, and takes advantage of Dark Leo's emotional distress to reassume control of the body. Good Leo carries Sadao to other Foot Ninja to treat his wounds, and quickly locates and kills an innocent Morinaga family asleep in their beds.
One month later, Karai visits Sadao, and informs him that Leonardo has since started regularly killing Morinagas with the rest of the Foot Clan, and has even helped them greatly speed up their mission; in addition, Dark Leo hasn't resurfaced at all. Sadao apologizes to Karai that he can't be of more help, but Karai tells him to rest up. Sadao then spots Leonardo passing the doorway, and speaks out to him, expressing open schadenfreude for Leo's suffering. But Leo is far more affected by the mere fact that Sadao is still alive. Leo rushes to Sadao's bed, and Sadao thinks Leo is about to attack him. Instead, to Sadao's alarm, Leo checks his pulse, relieved that finally someone survived Leo's dark side. Leo begs to see the living face under Sadao's always-worn Foot Ninja mask, but Sadao demands to know why. Leo explains the extreme torment he now feels from all the innocent people he's killed—Morinagas, bystanders, and even his own father; Leo hopes seeing the face of someone who survived his rampage could help ease his mind. Sadao looks upon Leonardo's thousand-yard stare and tremoring body, confirming the honest truth in Leo's words and the severe posttraumatic stress disorder that now inflicts him. Sadao soberly admits that he already knows all about the curse, its effects, and that the Leo he sees is not the same Dark Leo who killed his Ryōta. Sadao confesses that he should have accepted Ryōta's death as a casualty of the curse, but had refused to let himself believe it until now, adding that he keeps seeing Ryōta's murderer every time he sees Leo's face. It was easier and more satisfying for Sadao to blame someone else than to blame something as abstract as a curse. For the first time, Sadao feels affected by Leo's genuine suffering, realizing that it's just like his own suffering. Leo again begs to see Sadao's face, and Sadao sympathizes with the request, but still cannot bring himself to do Leo any favors as long as there exists a part of Leo who murdered Ryōta.
A week later, Leonardo and Sadao start killing Morinagas together. Because Sadao reasons that victims are more likely to scream in terror at a "naked kappa," Leo starts wearing a permanent outfit of traditional Japanese men's clothes. Over the next four years, Leonardo and Sadao carry a shared burden in their killings, helping Leo learn how to live with the carnage.
On one day after four years, the Foot Clan's extermination operations have reached the Japanese city of Sendai. Sadao reports to Leonardo that an entire Foot Ninja squad has committed group suicide and blamed Leo for it. Leo tells Sadao he wishes the Foot Ninja would complain to Leo directly instead of ending their own lives; he doesn't even care about being blamed anymore. Leo also tells Sadao that, to block his emotions, he simply thinks of the faces of his three Hamato brothers back in New York. Sadao relieves that Leo never had to see his face after all. But Leo asks again if he may see Sadao's face, still having never seen it after all this time. Sadao asks Leo why he would need to see it if he's already coping so well. Leo explains that his reasons are personal—now he is fond of Sadao and wants to get to know him better. Sadao becomes uncomfortable, sensing that "Leonardo" is trying to seduce him. Leo denies this, and Sadao reveals that he interpreted Leo's pulse check four years ago as an opportunity to grope his body, and doesn't want to imagine what Leo wants in seeing Sadao's face. Sadao explains that he is not attracted to Leo—or any mutant reptiles. Leo is surprised that, for the first time, Sadao has called him "Leonardo" instead of "kappa," and asks if that means Sadao hates him less. Sadao admits to it a little.
Over the years that follow, Leo comes to love Sadao, even as Sadao continually insists he doesn't find Leo attractive. Leo's love for Sadao is ambiguous in that it's never clear if he loves Sadao like family or is actually in love with him.
- Sadao is named after Sadao Hasegawa, a Japanese homoerotic artist who died in 1999.