In Zanpakuto, which allow them to perform their various functions, from guiding souls to the afterlife to killing the dangerous Hollows. While most might think a Zanpakuto sword is a Soul Reaper's greatest tool, there's actually something else that has proven to be a bit more essential: the Hell Butterfly.
Hell Butterflies, or Jigokucho in Japanese, are a special type of creature that the Soul Society makes extensive use of. They first appear in the very first episode of the series, long before Rukia has drawn her sword, and continue to appear throughout its run. The primary purpose of the Hell Butterflies is communication, as these beings are able to direct messages between the World of the Living and the Soul Society, allowing them to easily get in touch with their agents in the human realm. However, there's another ability that the Hell Butterflies have which really cements their usefulness.
Navigating the Border of Bleach's Afterlife
While spirits being dismissed and purified are able to make it to the Soul Society with ease, it's a little more difficult for Soul Reapers and living humans to get there, as a realm known as the Dangai - precipice world - lays between the two worlds. The Dangai can be incredibly dangerous, a warped maze that often resembles a long hallway. It's also equipped with a device known as the sweeper, which collects anything that might wind up in the Dangai and ejects it back out, at a random point in space and time. Obviously, that would be highly inconvenient for Soul Reapers on missions, so instead the Soul Society makes use of the Senkaimon, a portal that resembles traditional Japanese doors which Soul Reapers can use to enter the World of the Living or return to the Soul Society--but only if they have a Hell Butterfly to guide them. Without that, even a Soul Reaper trying to use the Senkaimon will be dropped into the Dangai precipice world.
The Hell Butterflies are also interesting in that they seem to be one of the few lifeforms that are actually native to the spirit world. Even Hollows, after all, begin as human spirits before being transformed. The Hell Butterflies are fully spiritual beings, too, made up of spirit particles and only visible to those humans with great spiritual awareness. It's unknown if they actually originated in Hell, the most mysterious location in the series that has become even more important after Bleach's anniversary one-shot manga, or if the name is merely meant to allude to their all-black appearance. As living beings, the Hell Butterflies do need to be cared for, a task which usually falls to lower-ranked Soul Reapers.
While a Hell Butterfly may not be quite as exciting as a Zanpakuto, they're still one of the most essential things a Soul Reaper in Bleach can have on them at any given time, and they could be the key to unraveling the mystery of Hell in the Bleach sequel that fans have been waiting for.
