The Guardians are the Seijin enforcers of The Sphere.
By Sphere law, all children with Seijin potential are taken by the state and learn to use their powers in the Guardian Academy, were they can better use their abilities without harming themselves or others. But the truth is, the Guardians are a means to keep rigid order over the Sphere-controlled blocks, and to expand the Sphere's control. They serve as police offices with absolute authority in most matters, commanders to their robotic army and as officers to non-Seijin personnel of the Sphere.
They started off with good intentions, but when Oslo took over, the Guardians became brutal enforcers and conquers, with Oslo effectively being the supreme tyrant of the Sphere. As learned in "Alice", members of the the Guardian Academy are brainwashed to be distrustful of all outsiders, that any dissent against the sphere is traitorous, and that all rebels (including sympathizers) are ruthless, evil pirates who must be destroyed at all costs.