WARNING: The following contains spoilers for Yasuke, now streaming on Netflix.
As a samurai, Yasuke has killed countless individuals, but there's one in particular that still weighs on him after 20 years. The Onna-bugeisha Natsumaru was once someone he counted as a friend, and even when she betrayed Oda Nobunaga's cause, Yasuke still felt respect and understanding for the woman he now had to kill. As Yasuke progresses on his journey, the memory of Natsumaru both haunts and ultimately saves him.
As retainers of Oda Nobunaga, Yasuke and Natsumaru were both outsiders fighting for respect through sheer talent. Yasuke faced racism as a Black man, while Natsumaru was subjected to misogyny as a woman. Their parallel struggles gave them a common ground for connection. Natsumaru further served to challenge Yasuke's perspective -- the African samurai was giving his all to fit into the feudal system, but Natsumaru was willing to question whether the system was worth upholding in the first place.
Natsumaru ultimately turned out to be a spy working on behalf of Nobunaga's opponent Hattori Hanzo. As a result of this betrayal, Yasuke was forced to kill her on the battlefield with her own sword. Not too long after that, Nobunaga committed seppuku as his army was defeated, leaving Yasuke without a master and with further doubts on whether the carnage that cost him his friend was worth it.
Both Yasuke and Natsumaru were following the same code of honor, and for that, Yasuke still has to respect his former colleague even if they were on different sides. Killing her is the single source of his greatest regret. In the series' climax, when the evil witch Daimyo tries to trick Yasuke into killing Saki, the magical girl he's on a mission to protect, the memory of Natsumaru assures him of his honor and his ability to make his own decisions.
The importance of Yasuke's relationship with Natsumaru is driven home in the series' post-credits. The six-episode Yasuke anime ends with him making a visit to Natsumaru's grave. Talking with her brother, whom he saved long ago, Yasuke says she died with honor and gives her sword to him.
Created by LeSean Thomas, Yasuke stars LaKeith Stanfield as Yasuke, Takehiro Hira as Nobunaga, Maya Tanida as Saki, Ming-Na Wen as Natsumaru, Paul Nakauchi as Morisuke, Darren Criss as Haruto, Julie Marcus as Nikita and William Christopher Stephens as Achoja. The series is now streaming on Netflix.