Zanpaku-to - It's unique shape and abilities are defined by its soul reaper. The soul reaper is able to harness the power of the zanpaku-to by learning its name and forming a synergistic unit.
The zanpaku-to is born with its soul reaper and dies with its soul reaper. That is the existence of a zanpaku-to.**
**Credit: Bleach DVD's Sets 16 and 17.