Azuki Rousai is a supporting character from the Iga Tsubagakure clan, selected as one of its ten representative ninja in a deadly conflict against the rival Kouga clan. He is notably the second oldest member of the Iga ten, surpassed only by Tenzen Yakushiji. Rousai has a distinctive and aged appearance, characterized by a very large, gourd-shaped head with a small tuft of white hair on top, a long white beard, and a sharply pointed nose. His face is marked with many wrinkles, and he possesses small, piercing eyes. He is the shortest of the Iga ninja and typically wears a yellow shirt, a red vest, dull blue hakama pants, and sandals.

His personality is defined by an extremely short temper that matches his short stature. Rousai harbors a deep and abiding hatred for the Kouga clan, a loathing that is both personal and historical. He claims his unusually large head is swollen with this hatred, stemming from the Kouga's surprise assault on the Iga in conjunction with Nobunaga's forces. He is easily irritated by teasing, particularly regarding his age, and is known to hold grudges against those who mock him. Rousai is deceptive and underhanded, having no reservations about lying to Oboro, the young princess of the Iga, and conspiring with other clan members to assassinate her betrothed, Gennosuke of the Kouga. He is also described as lecherous, showing no hesitation in molesting young women.

In the story, Rousai is a proactive and aggressive participant in the conflict. He is first seen confronting Jousuke Udono of the Kouga over a scroll, showing his willingness to break a long-standing peace treaty to attack an enemy. He joins Tenzen and other Iga ninja on a mission to hunt down and kill the Kouga representatives, successfully ambushing and eliminating Shougen Kazamachi. He later participates in a direct assault on the Kouga village, where he uses his unique abilities to dispatch multiple enemy ninja with ease. Rousai's role is ultimately cut short when he is tasked with interrogating Okoi, a captive Kouga ninja.

Rousai has significant relationships with several key characters. He views Oboro as a failure and unfit to lead the clan, showing her no loyalty and plotting behind her back. He holds an extreme hatred for Jousuke Udono, finding the Kouga ninja's cheerful and joking attitude, especially when the jokes are at Rousai's expense, to be utterly intolerable. He follows the leadership of Tenzen, the de facto field commander of the Iga team, though he is shown to be impatient with Tenzen's more cautious strategies. His relationship with the captive Okoi proves to be his downfall.

In terms of development, Rousai does not undergo a significant personal transformation. He remains a fiercely hateful and aggressive individual until his death. His most notable moment comes during the interrogation of Okoi. Losing his temper when she refuses to answer his questions, he places his hand on her shoulder to threaten her. He immediately finds his hand stuck, as Okoi possesses the ability to absorb blood through her skin. In a fatal error, he places his other hand on her as well, and she proceeds to drain the blood from his body, killing him and leaving his corpse shriveled and twisted.

Rousai's most notable ability is his elastic body, which allows him to stretch his limbs to unnatural lengths. He uses this power for long-range sneak attacks or, when surrounded, to sweep his extended arms and knock over multiple opponents at once. Despite his frail and ancient appearance, he is a highly skilled martial artist specializing in hand-to-hand combat. His punches and kicks are immensely powerful, capable of felling trees and smashing rocks with a single blow, and he can easily break an enemy's neck. He is never seen using any weapon, relying solely on his taijutsu and stretching abilities in battle.