You can't keep a good man down.
Saints are humans on a mission from God -- or at least, from one of His Archangels. They are souls who died having fulfilled their Destiny, went to Heaven, and then decided they couldn't stay while there was still evil to be fought on Earth. They are also sometimes called the Bodhisattva, after the Buddhist holy men who vowed to stay behind and help all men reach enlightenment before claiming it for themselves.
Saints are built on 7 Forces. They are often former Soldiers, but not always; ordinary people are just as capable of feeling the Saintly impulse! Actually receiving the Forces and going to Earth requires the permission of an Archangel, who will want to be sure that: 1) he or she needs a new Saint and 2) that the soul's motives for returning are completely selfless.
Once chosen, a Saint will be sent to Earth in one of two ways. Some are sent full-grown with an immortal vessel similar to that of an angel. Others are literally reborn as children, recover their memories sometime in childhood or adolescence, and live a normal lifespan. The first kind are usually sent with a specific mission, while the latter are often just expected to inspire others through a good life.
Either way, Saints can learn any Songs an angel is willing to teach and can be quite supernaturally effective, while still being able to avoid disturbance in the Symphony. A Saint can even assume a celestial form of sorts, when at a Tether. If they die on Earth, they are usually expected to enjoy their Heavenly reward; it is rare for an Archangel to send a Saint back twice.