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Isane Kotetsu (虎徹 勇音, Kotetsu Isane) is the lieutenant of the 4th Division, the medical and supply relief division, in Gotei 13 under Captain Retsu Unohana.

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 involving kamaboko (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 the Shinigami 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.