Kaiya Yamamoto (山本海也, Yamamoto Kaiya) is a third-year student at Matabi Academy and the Disciplinary Committee Chairman of the student council. He is the owner of Hisui.
Kaiya is a large and well-built teenage boy with spiky black hair and brown eyes. He is most often seen wearing his school uniform.
Kaiya has a friendly and laidback nature.
As a child, Kaiya was close friends with Kotori Hasutani. He was shorter and weaker than her at the time, so she always looked out for him. She told him to drink milk and jog every morning and he did so, resulting in his eventual muscular physique.
At some point during his time at Matabi Academy, Kaiya dumped a second-year student that was attracted to him, which Kotori witnessed. Kotori later asked him why he dumped her and replied that he was fine with things the way they were.
At the start of the series, Kaiya makes his debut appearing before the students at the first morning assembly of the new year, along with the rest of the student council. Once Tsukumoisshu attacks the school, Kaiya appears before him alongside the rest of the student council. Tsukumoisshu then summons a swarm of leaf minions that Kaiya and the others must deal with. The leaf minions disappear after Yumi and Kansuke defeat Tsukumoisshu.
The day after the battle, Kaiya is with the other student council members as they observe that Kaen's seal has been broken.
Two weeks after Tsukumoisshu's attack, Kaiya attends another meeting with the student council to discuss the apparent inactivity of the spirit beasts, despite the breaking of Kaen's seal.
The day after the meeting, Kaiya meets with Kotori at "The Hill for Confessing Love", as deemed by the girls at Matabi Academy. However, rather than a love confession, Kaiya simply asks Kotori to sew up his tattered uniform. Flustered, Kotori slaps him, but nevertheless agrees to fix his uniform. Kaiya then asks her about her battle with the boar demon and her attitude about the spirit beasts in general. Kotori says that she'd like to talk things out with the spirit beasts, but knows that a 1,000-year-old grudge will be hard to overcome. Kotori asks Kaiya how he feels and he replies that he hasn't thought about it, but knows what he needs to do. Kotori replies that he is acting strange. Kaiya agrees and then leaves.
Some time after the Rachi incident, Yumi and Kansuke are lured to the spirit beast Daraku. Kaiya becomes aware of this and decides to investigate, knowing that Kotori has also gone after them. He meets up with Shin at the entrance to the tunnel leading to the forest where Daraku is located. Using Hisui's ability, Kaiya flies over the tunnel to scope out the area, but is attacked by the spirit beast Hakubi who knocks him down into the forest.
After falling to earth, Kaiya and Hisui are confronted by Daraku's hoard of zombies. However, the zombies dissipate as Shin destroys the lead zombie that was giving them life. Kaiya then heads towards Yumi's and Kaiya's location and arrives just in time to save Kaiya as she falls off Musashimaru. Afterwards, the group concocts a plan to defeat Daraku. Using Kansuke as bait, Musashimaru pulls Daraku out of the water with a fishing line, after which Shin delivers the killing blow. Kaiya then sees the reflection of the moon in the water, indicating that the fog has cleared. However, Daraku, still alive, utters his final words, claiming that the barrier will be reversed. The group then sees smoke in the distance and Kaiya, taking Shin with him, flies back to Matabi Academy.
Kaiya and Shin soon arrive back at the academy, meet up with Futaba, and see the Kaen-possessed Tsubame emerge from a burning building. Kaen attacks them, but Kaiya manages to shield them from the attack. Kaen then taunts them by reciting personal information about them, which he reads from Tsubame's memories, and then shows them that the Futakago Barrier has reactivated. Kaen declares his intent to go through the barrier and kill Kirihime and Shirayuki, but Kaiya and the others try to stop him. While Futaba holds Kaen down, Shin attempts strike him, but Hakubi intervenes. Hakubi then attacks Shin and Kaiya uses his shield to protect him. Kaen then gathers the other spirit beasts and leaves through the barrier.
After Kaen leaves through the portal, Shin is determined to chase after him and orders Kaiya to take him through the Futakago Barrier. Kaiya complies and the two fly off. Once they reach the entrance, they see Tsukumoisshu descend from the sky.
Kaiya and Shin realize that the school vice principal has been possessed by the spirit beast and Shin to free him from his control. However, to their surprise, Tsukumoisshu releases his hold on the vice principal, sending him plummeting downwards, and knocks Shin off of Kaiya's air board. Shin tells Kaiya to go ahead and save Kirihime and Shirayuki. Though concerned, Kaiya decides to obey Shin and go on ahead.
Some time afterwards, Yumi, Kotori, Tsukasa, and Futaba also arrive in the Futakago Barrier. Along the way, they encounter an injured Hisui and Kaiya appears before them. He apologizes to Kotori, saying it's a long story, before transforming into Kibao who reveals that he has been possessing Kaiya's body. Kibao battles them, but they are easily overwhelmed. Kibao then unleashes his true power in a final devastating attack, but Kansuke stands his ground, calling his bluff, revealing that Kibao never intended to kill them. Hisui explains that Kibao and Kaiya only fought them to stop them from going any further to protect Kotori. Kibao then gives Kaiya control of his body again and Kotori slaps him and tearfully asks why he didn't tell her. Kaiya then explains how he met Kibao the night everyone else was fighting the spirit beasts. Though he didn't have Hisui with him, he fought Kibao, but was defeated, after which Kibao possessed him. Kaiya then returns control of his body to Kibao who explains the history of the spirit beasts and Kirihime. He also explains that he wishes to fight alongside humans, but also wants the spirit beast to be freed. Therefore, he will attempt to stop Yumi and the others until the spirit beasts are released.
Akitaka Sandou Arc
After hearing Kibao's explanation, Kotori intercedes stating that, while she sympathizes with the spirit beasts, she cannot allow the Futakago Barrier to be opened unless there is a truce. Since Kibao and everyone else agrees that will never happen, Kotori decides to stay behind and fight Kibao with Musashimaru while the others go ahead. Kibao speaks to Kaiya in his mind, claiming that Kotori is a stout-hearted girl worthy of protection, which Kaiya affirms.
Later, after Kaen kills Kirihime and Shirayuki, Kibao and Kotori stop fighting and rejoin the others. Since the spirit beasts have been released, Kibao upholds his agreement to help the humans however he can. However, Kibao and Kotori are intercepted by Hakubi who begins to battle them.
However, their fight is interrupted as the rubble from the island begins to swirl and come together at one point. Kiba leaves Kaiya's body and he and Kotori return to earth to join the rest of the student council. They go before a large temple that has appeared and witness the rebirth of Sandou, who hid his body among Tsukumoisshu's skull collection. The spirit beasts attempt to destroy Sandou, but he only absorbs their powers, making him stronger. Sandou then explains that six large pillars have been around since before the spark of life itself and are the source of Futakago's power, as well as all magic and spiritual power. He plans to use the pillars to return that power to the world and become the leader of the spirit beasts.
After Sandou finishes speaking, he abducts Yumi, and knocks Kaiya and the other student council members away. They soon awaken in a forest with Kibao and find that they cannot use their powers. Nevertheless, they are determined that they must do something to stop Sandou.
Kaiya and the others then discover an arrow that was shot by Tsubame that leads them to the magic formation that Sandou is using to conjure his powers. They destroy it, which enable Kansuke to deliver a killing blow to Sandou.
With Sandou defeated, Kaiya and the rest of the student council meet up with Yumi and Tsubame. Kaen then appears before them, stating that the spirit beasts have scattered and he will try to keep his existence a secret. The group then realizes they can no longer communicate with their cats. They are disappointed, but know it is for the best.
Three months later, Kaiya and the other students of Matabi Academy have relocated to an unused school and readjust to normal life.
Kaiya can produce a material that his cat Hisui can levitate and transform into various shapes. Kaiya often uses this to surf in the air, but he can also use it as a shield and various other items.