Takuma Zaizen represents the highest authority of the STN-J. His only superiors are the executives of Solomon Japan and, of course, Solomon Headquarters. Normally, he heads STN-J operations from his office at the STN-J's main building at Ravens Flat.

But some time before the story begins, he had created an auxiliary branch of the STN-J, known simply as "The Factory". On the surface, its design and purpose seem simple enough: to research and implement a more humane means to hunt Witches: in other words, to hunt Witches without necessarily killing them. With Zaizen's backing, the Factory came through with an experimental substance called Orbo. This strange green liquid has the ability to absorb and thus dampen Witch powers. It's used in two primary ways.

Most of the Hunters wear a pendant containing a vial of Orbo as a defense. Although the pendant dampens any Craft that may be in use (such as Karasuma's scrying), it provides an invaluable means of blocking almost any Witch power directed at them by absorbing the energy and prevents an attack from inflicting bodily harm on the wearer. This defense is limited, however, against a focused Witch power which can bypass or overwhelm the Orbo.

Orbo's other use is in offense. The Hunters can carry air guns with special bullets filled with the substance. Fired into a Witch, the Orbo enters the bloodstream and works to dampen the Witch's power and render him/her helpless. Once contained, the Witch can then be restrained, picked up by the Factory, and taken to confinement (Note: None of the Hunters understand the exact nature of this confinement; this will come into play towards the end of the series).

There is one problem with Orbo. For reasons unknown, Orbo cannot be safely handled by anyone except those with the Witch Gene. Fortunately, all Hunters are Seeds (unawakened Witches) at the least and so are capable of using Orbo. Robin is the only Hunter who doesn't use Orbo: for personal reasons, claiming it disgusts her. However, even as the series progresses Zaizen is continuing the research into Orbo, hoping to lift that limitation. At the same time, there are rumors that Solomon Headquarters isn't exactly thrilled with this prospect, and this may well lead to disagreement and even open conflict between the branches.

Touko (Robin's roommate in the first half of the series) is actually his daughter from a previous marriage (which is why their last names differ).