Warning: Spoilers for Bleach: The Thousand-Year Blood War, Episode 10
In Bleach, a Soul Reaper's Zanpakuto is almost like a part of their body, with rookie Soul Reapers expected to bring their sword with them everywhere they go. The process of creating a Zanpakuto is actually a lot more complex than that, however, and that's from the original creator of the swords himself.
To join Squad Zero, also known as the Soul King's Royal Guard, it's necessary to have left a big impression on Soul Society history. Among its five members is Oetsu Nimaiya, who is revealed to have been the first Soul Reaper to craft a Zanpakuto, and is the one who seemingly still creates every blade for the Soul Reapers to this day. Since Ichigo and Renji have seen their Zanpakuto take some major damage lately, meeting the original creator is about the best-case scenario for getting them fixed, and with that comes a lesson on how exactly those swords are made.
In episode 10, Ichigo and Renji meet Nimaiya, a rather eccentric guy who speaks partially in English and surrounds himself with beautiful women. As it's soon revealed though, these women are actually the spirits of Zanpakuto. Each Zanpakuto sword has, as its base, a kind of highly mutable spirit known as an Asauchi. These Asauchi are transformed into the generic Zanpakuto that the average Soul Reaper wields, with neither a Shikai nor Bankai transformation yet available. As the two spirits spend time together, the Zanpakuto will begin to adapt and take on a form better suited to its wielder, which is how the high-ranked Soul Reapers have such specialized weapons. Nimaiya tricks Renji and Ichigo into walking into a cabin with a large pit, full of angry Asauchi, and says that they must survive in order to claim a new sword. At the end of this trial, he reveals that the purpose of this was to allow one of the Asauchi to choose them and become their new Zanpakuto base. Apparently, Ichigo hasn't had one at all in his blade, and he still won't, as Nimaiya refused to make him one.
Creating a Zanpakuto is Personal
The idea of Zanpakuto having a spirit inside them has existed since the beginning; when Renji was shown communing with Zabimaru to obtain his Bankai, his blade's spirit, it took on the form of a baboon with a snake for a tail. The idea was further explored in an anime-exclusive filler arc, where Zanpakuto spirits took on humanoid form and rebelled against their masters. The Asauchi do indeed have a humanoid form, whereas the Zanpakuto women are almost indistinguishable from an ordinary human soul. Additionally, whenever Ichigo has spoken to Zangetsu, he was always been shown with a human form, either the "old man" or a negative-color version of Ichigo. Knowing that Ichigo was without an Asauchi certainly raises some questions about Zangetsu's true nature... if only Nimaiya would be willing to answer them.
It makes sense for something so personal to a Soul Reaper to have a spirit of its own in Bleach, such that even when fighting by one's self, they're never alone.
New episodes of Bleach: The Thousand-Year Blood War currently premiere on Mondays on Hulu.