Yakumo "Cloud" Katou (加藤 "クラウド" 八雲 Katō "Kuraudo" Yakumo?) is AoNinger (アオニンジャー Ao Ninjā?, Blue Ninja), who is known as The Cool Ninja (おちついた忍者 Ochitsuita Ninja?), the Blue Ranger of the Ninningers. He is mostly nicknamed as Yakkun (やっくん?) by Fuuka and Nagi.
One of Yoshitaka Igasaki's successors, Yakumo 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 return to Japan. 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. In the end, Yakumo was forced to abandon his magic training from England to take the mantle of AoNinger.
During the hottest point in summer, the boys decided to play a prank on the girls after Nagi overheard Kasumi confide in Fuuka that she was afraid of ghosts, with Yakumo only agreeing after being accused of being scared of Kasumi. Luring the two to an abandoned building which they claimed to be haunted possibly due to a Yokai, they proceeded to put on spooky costumes, with Yakumo dressing as a fallen soldier. Pulling their prank, they were confused to see only a spooked Fuuka run out, especially due to her claiming to see a 'lady ghost' which they did not account for. Takaharu headed back for Kasumi, followed by Yakumo and Kinji. With no word, Nagi and Fuuka found they had been frozen by the Yokai Yuki-Onna, with Fuuka soon following suit. Left alone, Nagi managed to conjure some heat as Kasumi revealed herself having used a decoy, luring the Yokai into a trap. Yakumo and the others were thawed by a Fire technique as the Ninningers proceeded to defeat Yuki-Onna
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 Yakumo and Nagi disguising themselves as potential actresses alongside Kasumi. Along with all the other candidates bar Fuuka; Yakumo, Nagi 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. Yakumo and the others soon wake up when their life energy is returned following Dracula's defeat by Fuuka, Takaharu and Kinji. Yakumo, Nagi, 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, Yakumo was disappointed to learn that he had only made second-from-last at 5th. 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, Yakumo along with Nagi, 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. This intriuged the Ninningers, with Yakumo sneaking into the school with Kasumi and Kinji, they learned that Kiroku was involved with the Kibaoni Army following a skirmish with Masakage Tsugomori, warning Takaharu who came to confront him personally. Yakumo 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 AoNinger, Yakumo fought through the Yokai horde 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 Magic Ninja, Yakumo returned to his magic school in England where he taught a combination of ninjutsu and magic.
Yakumo has lived most of his life abroad and was learning European magic in England when he was called back. He is the rich boy type with a cool demeanor and somewhat presumptuous. In contrast, however, he has a quick temper and can be a bit of a bonehead at times. Out of his cousins/Ninningers, Yakumo is a quick-learner and can master ninjutsu quickly than the others, claiming it easier than magic.
Yakumo at first had no interest in Ninjutsu, claiming it easier than magic training. This was however until when he was lectured by Takaharu/AkaNinger and had developed a rivalry with him, he decided to compete with the others to inherit his grandfather's title as the Last Ninja.
Thanks to his tactical mind and maturity, he is more level-headed than his cousin Takaharu, and as such can be seen as the unofficial leader of his Sentai team. After bonding with Kinji Takigawa/StarNinger, Yakumo's partnership with the American-born ninja is jokingly refered to by the other Ninningers as the Western Combo (欧米コンビ Ōbei Konbi?, Western Combi), coined by Kasumi, refering to the special relationship between the United States of America and the United Kingdom of Great Britain.