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Nagi Matsuo (松尾 凪 Matsuo Nagi?) is KiNinger (キニンジャー Ki Ninjā?, Yellow Ninja), who is known as The Agile Ninja (身がるな忍者 Migaruna Ninja?), the Yellow Ranger of the Ninningers.
One of Yoshitaka Igasaki's successors, Nagi was one of his grandchildren chosen to become the Ninningers. He was called by Tsumuji Igasaki, his uncle and Takaharu and Fuuka's father to their dojo, which much later turned into a pile of ruins by Hitokarages. After reunited with the other Ninninger candidates, Tsumuji leads them to Gengetsu Kibaoni's resting place, which was sealed by Yoshitaka until the Sealing Shuriken were released by the revived Gengetsu and wandered off to various part of the city. After the Ninningers destroy Youkai Kamaitachi, it grew large and they received five OtomoNin Shuriken, which they used to form Shurikenjin and finished the Youkai. Yoshitaka revealed himself to be much alive, but lectured them for their inexperience and lead them to their new dojo that they will use as their new base.
Nagi at first tried to use textbooks as guidance in his fight but after his near-death experience by Youkai Kasha, he decided to use experiences instead, taking Takaharu as an example for not thinking over his actions first.
Alerted to a Youkai appearance, the Ninningers discovered only a Sealing Shuriken found by Fuuka, realizing that someone else was now fighting the Yokai. Following this, it was revealed that Yakumo's mother, fashion designer Harukaze Katou, had the End Shuriken as an accessory on a designer dress she made for her new fashion show in Japan. The Ninningers then enacted their plan to recover the End Shuriken and swap it with a fake, with Nagi disgusing himself as a security guard taking a shift to let Takahru and Yakumo in. However, matters were complicated as the Kibaoni Army moved to take the End Shuriken themselves, sending the magic Youkai Ittan-momen and the new Jukkarage grunts. His first act was to use a transformation spell to turn Fuuka into a mouse, with Kasumi seemingly following, leading Takaharu to house them in a mouse cage. Nagi was then turned into a rock before the Ninningers caught up with the End Shuriken, with Yakumo being the last one standing when Takaharu was turned into a doll. However, he was able to outsmart Ittan-momen with a combination of magic and Ninpou as Kasumi, having used a decoy, acquired the End Shuriken. Nagi and the others were then changed back to normal when Yakumo used a reverse spell against Ittan-momen. The Ninningers found that the End Shuriken was a fake all along when Kyuemon attempted to take it before receiving unexpected aid from a sixth Ninninger with his own OtomoNin in the giant battle.
When Fuuka was scouted by a producer for his new movie featuring the popular actor SILVER as Dracula, the Ninningers, suspecting the Western Yokai Dracula was involved, went undercover and accompanied Fuuka to her audition, with Nagi and Yakumo disguising themselves as potential actresses alongside Kasumi. Along with all the other candidates bar Fuuka; Nagi, Yakumo then Kasumi as well fall victim to Dracula's bite, putting them into a deep sleep allowing him to harvest human fear through induced nightmares. Nagi and the others soon wake up when their life energy is returned following Dracula's defeat by Fuuka, Takaharu and Kinji. Nagi, Yakumo, and Kasumi then board their OtomoNin to get back at Dracula, pushing him back enough for Takaharu to destroy the enlarged Western Yokai.
When Yoshitaka announced the mid-point results of the Last Ninja race, Nagi was pleasantly surprised to learn that he had made 2nd place. In reality, however, "grandad" was actually Nekomata's doppelganger Yokai Mataneko in disguise, who intended to get revenge against the Ninningers by breaking them up, hosting a lightning quiz round when the initial results failed. After the Ninningers noticed signs that something was amiss, Kasumi fetched a second Yoshitaka, exposing the first as a fake via catnip. The Ninningers then transformed to fight him, with Takaharu as AkaNininger Chozetsu defeating Mataneko with Shinobimaru after being accidentally enlarged alongside the Yokai. Afterwards, at the request of his grandchildren, Yoshitaka gave his true mid-point results, stating that they were all still at the starting line, with this news only giving them a strengthened resolve to continue training.
Finding Takaharu bringing a girl to the house, Nagi along with Yakumo, Kasumi and Kinji, discussed this new development while eavesdropping on the two, wondering what Fuuka would make of her brother having a girlfriend. Inadvertently revealing themselves when Fuuka arrived, it turned out that Takaharu's 'girlfriend', Kikyo Kousaka, was an old friend of his from his days travelling while training as a ninja, having returned due to the opening of a new ninja school by the ninja master Kiroku Ise, where she was to instruct new students. However, Kiroku was being manipulated by the Puppet Ninja Kuroari who intended to recruit ninja initiates to the Kibaoni Army. Nagi and the others later helped the students escape from the school before saving Takaharu, Kikyo and Kiroku from Puppet Ninja Kuroari who had been manipulating Kiroku, with the Ninningers proceeding to defeat the dark ninja. After Takaharu said goodbye to Kikyo, the Ninningers were amused that he misunderstood what Kikyo meant by wondering what they would be when they next met, assuring him that he didn't have to worry about it yet.
Beyond KiNinger, Nagi fought through the Yokai horde alongside Fuuka in Shurikenjin before reuniting with the others as they backed-up Takaharu against Sakuya Kyuemon. With their unwavering resolve, all six Ninningers were able to restore their Nintality, transforming together and overcoming Sakuya Kyuemon, managing to get through to him before he was absorbed by Gengetsu who, as a giant, would be finally destroyed by the Ninningers in Gekiatsu Dai-Oh with help from Kyuemon, who left behind the End Shuriken before passing away. With the End Shuriken in their grasp, the Ninningers used it to wish for a world without the End Shuriken, finally wiping out the Yokai and restoring the damage that had been done to the world. With the title of Last Ninja passed onto them, the six Ninningers elected to go their seperate ways and meet back in two years to recollect what they have each done.
Naming himself the Last Support Ninja, Nagi studied to get into college while providing a helping hand to his classmates.
Nagi is a very friendly young man. He may seem to not be thinking of anything but is actually very observant and street smart. He is obsessed with getting licences and certifications for different things to make good use of. Knowing of his grandfather's title as the Last Ninja, he decided to compete against his teammates to inherit it, thinking the title would help him in looking for jobs. 
Like most Ninningers except Takaharu, he initially refused to become a Ninninger, having no courage until Takaharu encouraged them. At first, Nagi tried to use textbooks as guidance, but none of them helped him in his battle against Youkai Kasha. After that, he decided to use his experience in battle and instinct instead, taking Takaharu as an example.