Isane Kotetsu, 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 seats in the 13th Division. Unlike Kiyone, she is generally quiet and observant, though she shares her sister's sense of loyalty. 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 fish paste. As a pun, her given name contains the kanji for "brave," while her sister's contained the kanji for "quiet."

Isane's zanpakutō is Itegumo (凍雲, lit. Frozen Cloud, Viz: Frozen Snow). Its shikai command is "sprint" (奔れ, hashire?, English "run"). The guard is shaped like a snowflake. When its shikai is activated, two smaller blades protrude from Itegumo's hilt at 45° angles. She has yet to be seen using it in combat, since the one time she activates it she is immediately knocked out by Ichigo.