Nenji Kashiwazaki, known to his associates as Okina, is a formidable fighter and a talented strategist. He was once a leading member of the Oniwabanshu, the intelligence network that served the Tokugawa shogunate, and during the Bakumatsu he commanded the Kyoto branch in their efforts to monitor revolutionary activity in the city. When the Edo period ended, Okina spearheaded the transformation of the Oniwabanshu headquarters, the Aoiya, into a safehouse and inn where his comrades could find refuge. Although he was the expected successor to the Oniwabanshu leadership, he refused the position, believing it was time for the younger generation to take charge, and instead recommended Shinomori Aoshi to become the next leader. He then served as an agent in Kyoto, gathering information, and after his retirement he largely gave up his fighting ways and settled into a peaceful life running the Aoiya, where he raised Makimachi Misao.

Okina has a friendly and easygoing demeanor, with a love for life expressed through a taste for drinking games and the company of attractive women. He is known for his gentle yet excitable spirit and a fondness for celebrations and parties whenever the chance arises. Despite his carefree outward manner, he possesses a quick and agile mind. He knows the inner and outer workings of Kyoto intimately and still maintains the old Oniwabanshu intelligence network, which he uses to provide critical assistance to Himura Kenshin during his mission against Makoto Shishio. Okina can be stubborn, and he sometimes forces his help upon others even when they hesitate to accept it. He is an excellent listener with the ability to see through people's emotions, understanding when someone is trying to avoid causing trouble for others.

Okina's primary motivation in the story is to protect Kyoto and support Kenshin in his efforts to stop Shishio's plans. He offers Kenshin shelter at the Aoiya and activates his old spy network to help Kenshin locate the swordsmith Arai Seiku and the master Hiko Seijuro. At the same time, Okina is aware of the threat posed by Shinomori Aoshi, his former pupil and the current Oniwabanshu leader who has become obsessed with defeating Kenshin to prove himself the strongest. Okina warns Kenshin about Aoshi's intentions and ultimately chooses to confront Aoshi directly during the final battle to prevent him from interfering with Kenshin's fight. This confrontation leads to Okina's defeat, as he is gravely wounded by Aoshi.

Key relationships define Okina's role in the narrative. He raised Misao and treats her with great affection, and she in turn respects him as a guardian. His relationship with Shinomori Aoshi is complex: Okina recommended Aoshi for the leadership and was once his mentor, but Aoshi's path has turned dark, forcing Okina into the role of an opponent who must try to stop him. With Kenshin, Okina is an invaluable ally, providing resources, information, and a safe base, while Kenshin earns his respect through his commitment to protecting others.

Okina's abilities reflect his background as a senior Oniwabanshu operative. He is said to be the only person who can match the former Oniwabanshu leader in combat. He wields steel tonfa that are strong enough to block sword blows, demonstrating that despite his age and retired status, his fighting skills remain considerable. He also maintains his strategic and intelligence-gathering talents, keeping the old Oniwabanshu network in working order and using it to provide critical aid to Kenshin and his allies.