Bleach's massive cast of characters is full of colorful personalities, though many are overshadowed by the likes of Ichigo Kurosaki, Rukia Kuchiki and the like. Still, it's the multitude of background characters who help flesh out the world of Bleach, and that includes Soul Reapers such as Isane Kotetsu.
Isane is a fairly easy character to overlook. But while the Lieutenant didn't have a massive impact on the story, characters like her round out the world and backstories of more major characters. Isane, in particular, has a deceptively important role to play.
Who Is Lieutenant Isane Kotetsu?
Lieutenant Isane Kotetsu belongs to Squad 4, the medical corps, and she and her Captain appeared at roughly the same time in the Soul Society arc of the anime and manga. Isane, by rule, was an enemy to Ichigo and his infiltration squad, but only out of duty, unlike the battle-obsessed Captain Kenpachi or the sadistic Captain Kurotsuchi. Isane, however, hardly seemed to care at all. For one thing, as a dedicated healer and practitioner of defensive kido, Isane wasn't expected to fight off the intruders. Isane's battle with Ichigo actually lasted no more than a few seconds. Rather than tangling with Ichigo and co., Isane and her Captain were more concerned about Captain Sosuke Aizen of Squad 5.
Captain Unohana and Kotetsu were consulted when Aizen was seemingly murdered during the Soul Society arc. However, Unohana suspected that this was not Aizen's real body, to Isane's shock. Her Captain was proven right when they crossed paths with an openly villainous Captain Aizen near Central 46's chambers. Isane used her best kido to try and contain Aizen, but being a kido master himself, Aizen escaped. With this, Isane was convinced that Aizen, not the ryoka, was the true threat to Soul Society, making her one of the first to see him coming. But there was little she could do.
Sometime later, during the Hueco Mundo arc, Isane and Unohana both fell into Aizen's trap. The pair arrived in Las Noches to heal Ichigo and his friends until their Garganta portal was forced closed by Aizen's will. While Isane panicked, Unohana was confident in their eventual victory.
Later still, during the fight against the Wandenreich, Kotetsu took part in the counterattack on the Sternritter, despite the recent demise of Captain Unohana. Isane supported the young Lieutenant Yachiru Kusahishi of Squad 11, but she was cornered when Gremmy Thoumeaux's creation, Guenael Lee, attacked. In the end, Isane was saved when Yachiru won the battle and freed her to continue healing Squad 11's Soul Reapers. With or without her beloved Captain, Isane is able to carry out her duty without complaint.
How Isane Kotetsu's Abilities Work
Isane Kotetsu is an advanced kido user, an essential skill for any member of the medically-focused Squad 4. Isane can create defensive barriers around a wounded patient and heal them, just like her Captain. She can also perform advanced Bakudo techniques as well, as seen in her clash with Aizen. In terms of weapons, Isane has a fairly conventional Zanpakuto named Itegumo, and when released, it has two smaller blades that are at a 45-degree angle to the main blade. However, it's unclear what Itegumo's abilities are. It is likely that this Zanpakuto has healing properties.
Isane has one known family member, her younger sister Kiyone, who is a member of Squad 13. Kiyone is quite different from her older sister, being a hot-headed Reaper who constantly competes with Sentaro Kotsubaki for Captain Ukitake's favor. In her personal life, Kotetsu is a member of the Soul Reaper Women's Association, which is responsible for designing Reaper tools like the Soul Candy dispenser. It's unclear if Kiyone is also part of the Association, though.