Isane is a tall, young looking woman with gray eyes and short messy silver hair with strands on the right side of her face that are shoulder-length and styled as two thin braids. Some of her hair shoulder-length and is braided on the right-side of her head. She wears a thin dangling red earring on each ear and a standard Shinigami uniform with her lieutenant badge on her left arm.
Isane is the older sister of Kiyone Kotetsu, one of the two 3rd seated officers in the 13th Division. Unlike Kiyone, she is generally quiet and observant, though she shares her sister's sense of loyalty. However unlike her captain, she can be impatient in the face of an enemy, rushing after enemies she has no chance of defeating, including the seemingly unstoppable Sōsuke Aizen. She looks to Unohana as a mentor and mother figure. It is noted by Unohana that Isane suffers from frequent and recurring nightmares. This is sometimes used as material for jokes, as Isane's nightmares are often odd or whimsical in their contents. For example, she once had a nightmare involvingkamaboko (a fishcake made of pureed surimi), which she intensely dislikes. Her favorite food, on the other hand, is porridge, to the extent that she is capable of eating it as the three main meals on the same day.
Isane's exceptional height for a female Shinigami is often material for jokes. She started eating porridge when she passed 170 cm in her growth, because she did not want to get any taller (she believes - wrongly - that the nutrient level of porridge is very low). Unfortunately for her, she did not stop until reaching 187 cm, her height being another source of frustration for her. Her height is also mentioned on one of the assemblies of the Shinigami Women's Association, during which she proposes a cell phone sturdy enough to not break if it is dropped from a height of 2 meters, only to be told by Nanao Ise that it is not a common problem. Once, after hearing of how sleeping helps children grow, she decides to not sleep again in order to halt her growth.
When Isane has time off, she goes to see Kiyone, and vice-versa. They're very close sisters who enjoy meeting up and having fun together. Recently, her favorite place to hang out is the Kuchiki estate, in the "secret base of theShinigami Women's Association". She is a notable member of the latter Association, where her meek nature occasionally gets her in trouble with group activities including getting captured alongside Nemu Kurotsuchi when attempting to take a picture of Byakuya Kuchiki.
In a recent omake, Isane is shown to have a decent relationship with Jūshirō Ukitake. Ukitake gets lost in the 4th squad's headquarters and Isane helps guide him around. They run into Shunsui Kyōraku who was not as lucky, as he got lost and stumbled into Unohana's bedroom.
Isane is mostly seen around Retsu Unohana. When Hanatarō Yamada went back to the 4th division after he met with Jūshirō Ukitake, he gets in trouble with his Captain. However, when Isane is walking with him afterwards, Hanatarō asks if what he had done was wrong, and Isane answers he was brave for what he did.
Isane attends the execution of Rukia Kuchiki with her captain on theSōkyoku Hill as well as her rescue by Ichigo Kurosaki. She appears upset by Genryūsai Shigekuni Yamamoto's false promise to allow Rukia's would-be rescuers to leave safely after her execution, but Unohana tells her that he said it so she could die in peace. After Rukia is handed off to Renji Abaraito get away, Isane hesitantly joins with Chōjirō Sasakibe and Marechiyo Ōmaeda after them. She releases her Zanpakutō to fight against Ichigo Kurosaki, but is taken down much lighter than the other two. After her defeat, her sister Kiyone runs over to aid her, but Suì-Fēng intercepts her and knocks her to the ground. If not for the fast action of Yoruichi Shihōin, Kiyone would have been killed.
Unohana picks up Isane, and when she starts to apologize for failing to defeat Ichigo, Unohana tells her to calm down. After dropping the wounded off, Isane and Unohana quickly head to Central 46. They arrive at the scene in time to see Aizen defeat Tōshirō Hitsugaya. Captain Unohana announces her presence to the traitors while citing her own suspicions surrounding Aizen's staged death. She also goes on to claim that the Offices of Central 46 Chambers was the only place he could hide and mentions the use of an elaborate corpse doll on his part. Aizen then points out the mistakes in her analysis, while also commending her for seeing through his deception. Isane learns of Sōsuke Aizen's betrayal, and reveals that she and the other lieutenants thought Kyōka Suigetsu was a running water-type Zanpakutō, not one with the ability of perfect hypnosis.
Isane prepares to attack Aizen, but he and Gin Ichimaru quickly get away before she can get near them. Unohana then instructs Isane to use herKidō skills, both in tracking down Aizen and communicating with the remainder of Soul Society on the emergent schemes of the traitorous captain, as well as the Ryoka.
After the end of the incident, Isane assists the rest of her squad in healing the wounded. In the middle of the night, she wakes up while having a nightmare about fish paste. She wanders out to get some air while trying to decide what to do, only to be discovered by her Captain, who gently tells her to get some sleep..
Isane appears with Momo Hinamori at the 10th Division office, surprised to find Rangiku's Zanpakutō sticking out of the ground. When Rangiku tells her that she is trying to communicate with her Zanpakutō, Isane tells her that her own Zanpakutō, Itegumo hasn't been responsive lately, something that has never happened with her before. Later, she is summoned along with most of the other high-ranking Shinigami to Sōkyoku Hill as Muramasa and the other manifested Zanpakutō spirits appear before their former masters, and is ordered to treat Chōjirō Sasakibe after he is injured by the Zanpakutō. She and her captain are later visited by Nemu with a request by Mayuri Kurotsuchi that Unohana come to the 12th Division barracks. After Rukia is injured by Sode no Shirayuki, Isane is seen treating her as she wakes up. When Rukia asks about Ichigo's condition, Isane mentions that his location is unknown and no one can go looking for him with the current situation. However, Ichigo eventually returns on his own after subduing Zangetsu. She appears with Unohana later as she goes to tend the wounded. Though she tries to say something to her captain, she decides not to and leaves.
Later, she is out in the Seireitei, informing Lieutenant Tetsuzaemon Iba that Suì-Fēng and the Onmitsukidō have closed in on the Zanpakutō spirits in the 6th Division's barracks. As they both decide assist them, they are confronted by Ashisogi Jizō. It tries to attack them with a blast of poison gas, but she uses a ShōKidō spell to knock it away, and Tetsuzaemon uses his sword to cut down a building and trap it underneath the rubble. They try to leave to inform Mayuri about the situation, but are confronted by Haineko and Tobiume before they are able to leave. Ashisogi Jizō recovers as well. Isane tries to hit it with a Shakkahō Kidō spell, but it manages to avoid it and attacks her with several blades bursting through its front. Isane barely manages to block the blow, but is knocked backwards. While she tries to recover, she realizes that she can't move her legs. Tobiume explains that when Ashisogi strikes a limb, it becomes immobile, and that since Isane can't move her legs, Ashisogi Jizō's attack must have struck both of her legs. Despite her immobility, she tries to fight back against Tobiume, but the Zanpakutō spirit avoids her attack and tries to retaliate. However, Momo Hinamori arrives to block the blow.Rangiku Matsumoto arrives as well and attempts to fight Haineko, but Ashisogi Jizō confronts them and spews a large cloud of poison at all four of the lieutenants. Tetsuzaemon and Rangiku then help an injured Isane to run away along with Hinamori, but Ashisogi Jizō assumes its Bankai form and pursues. Even though they manage to escape, the four lieutenants collapse, affected by the Zanpakutō's enhanced poison. Isane is later seen in the 4th Division Headquarters, receiving treatment.
Isane comes into Nanao Ise's temporary office and offers her a box of cookies that Momo Hinamori made. It when she is about to make tea for the two that she notices that the tea leaves have vanished. She and Nanao then report this piece of information to the rest of the captains.
Having been locked away by Kagerōza Inaba, Isane is freed by Yoruichi Shihōin and carries an injured Akon to the confrontation with Inaba. She later joins the other lieutenants in fighting the Reigai with the captains. Despite evening out the numbers, Isane and the other Shinigami are still pressured by the Reigai.
Kidō Expert: Isane is shown to be highly proficient in Kidō spells, apparently excelling in chaining multiple spells together for various effects. While being the second most proficient in healing Kidō in the 4th Division, she has good understanding of binding spells as well. She is also shown using Hadō #1 Shō and #31 Shakkahō, revealing that she is skilled in the use of destructive spells as well.
Expert Healer: As the lieutenant of the 4th Division, Isane excels in healing techniques, being second in the 4th Division only to her captain. She is known for speedily providing medical attention.
Shunpo: Isane is proficient enough in to keep up with her captain.
Itegumo (凍雲, "Frozen Cloud"; Viz "Frozen Snow"). In its sealed form, Itegumo takes the form of a katana with a deep blue handle and a pointed round guard, somewhat similar to a snowflake. It is kept sheathed on the left side of Isane's waist.
Shikai: Its Shikai command is "Run" (奔れ, hashire). When its Shikai is activated, two smaller blades protrude from Itegumo's hilt at 45° angles.