A Reliever is a minor Heavenly spirit. They are typically assigned to assist angels, or perform assorted small tasks. Many Roles are diligently created by Relievers.
A Reliever can be created with as few as 3 Forces, one in each category. They do not necessarily have vessels; many simply flit about in Celestial form, rendering them effectively invisible to most humans. Their Celestial forms are tiny fairy-like creatures. When they reach 9 Forces, they "fledge" into full angels. From there, they can advance as far as any other angel in Heaven -- in fact, Zadkiel, the Archangel of Protection, is known to have started as a Reliever many centuries ago.
7-Force Relievers, like the ones used as PCs, generally have more significant tasks than the usual fetching car keys from between the cushions and such. They're being prepared for becoming full angels, and while they're always supervised on Earth, they can take part in more serious missions.
At any given moment, there are about 500,000 Relievers on Earth, although they come and go quite quickly. The average Reliever has only three or four Forces, and handles only very simple tasks.