

Kaien asked the Hollow how many Shinigami it had eaten and if it had ever felt sadness or regret about killing. Upon seeing the Hollow, Kaien asked for permission to fight it alone, which Ukitake granted. Ukitake and Rukia accompanied him to the Hollow's lair. Despite not knowing the name, appearance or powers of the Hollow, Kaien insisted upon going after it rather than waiting for an assault squad to be formed. When Miyako was killed by the Hollow, Metastacia, her body was brought back. Kaien was married to the third seat of the 13th Division, Miyako Shiba. He explained about his beliefs of the "heart" to Rukia and told her not to die alone, as when one dies, the heart is left with one's companions, where they would live on. He explained Ukitake's philosophy about there being two types of battle to Rukia, but then told her that he thought that both meant to protect the "heart".

He told her that it was to fight and protect. One day while training there, Rukia asked Kaien why she was in the Gotei 13. Koifushi in western Rukongai on a number of occasions. Kaien asked them if they were drunk and sent them back to their posts. They were interrupted by Kiyone Kotetsu and Sentarō Kotsubaki, both whom praised Kaien's coolness. He told her that as long as she was in his Division, he would stand by her even if he died. One day, Kaien brought Rukia a drink and asked her why she looked so troubled. He explained that due to Ukitake's illness, he was usually in charge and told her she could call him "Captain Kaien" if she wished. He introduced himself to Rukia and scolded her over her meek reply, treating her like any other Shinigami. When Rukia Kuchiki joined the Division, he scolded other members of the Division who were watching the newly arrived noble. Five years after graduating from the Shin'ō Academy, Kaien became the 13th Division's lieutenant. He noted that he was even further from being a lieutenant and asked which Division Gin joined. Ukitake compared his age to Byakuya Kuchiki, whom Kaien started to curse before saying that Gin sounded quite fearsome. Kaien denied being a genius and Ukitake told him that Gin graduated in a single year, which impressed Kaien. After Kaien declined, Ukitake mentioned Gin Ichimaru's graduation from the Academy and noted that he was the first genius since Kaien. Ukitake coaxes a reluctant Kaien to be his lieutenant.ġ10 years before the Ryoka Invasion, Kaien was approached by Ukitake, who tried to persuade a reluctant Kaien to become his lieutenant.

He was shortly recognized as a prodigy, as he completed the six-year curriculum of the Shin'ō Academy after only two years, and became a member of the 13th Division. Kaien later decided to become a Shinigami and he passed the entrance exam on his first try. The door guards of the Shiba family - Koganehiko and Shiroganehiko - were previously the educators of Kaien and his siblings. He gave the piglet, Bonnie, a ribbon and she became very fond of him. While in a mountain area in Rukongai, Kaien found an injured piglet and looked after it. Together with his siblings, Kūkkaku and Ganju, he grew up in Rukongai. Kaien was a member of the Shiba Clan when it was one of the Great Noble Houses. Kaien's favorite food was ohagi and he liked to take afternoon naps if he could. Having held such views, he believed that people should never die alone. He believed that one's "heart" exists in others, and that it remains with his or her comrades even after death. He thought of "heart" as not something inside an individual, but rather what is born whenever one thinks of or remembers someone else. Kaien took it a step further and believed that they both ended up being the same thing: "Heart." He believed that every time someone connects with another, a little bit of "heart" is born between them. He was a firm believer in Captain Ukitake's philosophy on fighting, which consists of two types of combat: the fight to protect life, and the fight to protect honor. Kaien believed that one's "heart" was determined based on the prospects of fighting and protecting. Kaien loved the Shinigami, never speaking out in hatred against them. Because of this, he was popular among aristocrats and commoners alike within his Division. Kaien treated his fellow company members indiscriminately, regardless of their social standing or rank. He was outspoken and bore a rough disposition, with a down-to-earth attitude. Kaien, a very ethical man, was adored by many members of the 13th Division. Some Shinigami, such as Byakuya Kuchiki, have stated that Kaien bears a strong resemblance to Ichigo Kurosaki. He had a tattoo of the Shiba family crest on his arm. He wore the normal uniform with his lieutenant badge on his left arm as customary. He had aqua green eyes and spiky raven black hair. Kaien had a youthful appearance and was a fairly tall person.
