Some say that one of the greatest bonds is shared between a person and the object they use most. For an author, that would be a pen. For a musician, it would be their instrument... but for a soul reaper it would definitely be their blade.
Blades are much like hollows in some regards. They are made from compressed souls in order to fight the evil, avenge the fallen, and most importantly protect the innocent. It is said that a soul reader's blade shapes themselves to their master's desire, and also to their fighting style. It is also believed by many that a Zampacto can never become stronger than it's master, but instead they will stand on th e same even footing until they both perish.
This however, is the average not the rule. A certain ginger soul reaper has managed to defeat his blades multiple times, and others have gotten consumed by them. Also, the swords crumble if they do not get a new master immediately after their master's death. Oddly enough, the sword looses all of it's previous personality, shape and abilities. Why? Because the embodiment follows the master.
Even when another sword's ability changes their allegiance completely. Still, they follow their master's to the living, the dead, hollows, and far beyond that.