It is their holy quest to save earth from the Oni. They are mystic warriors, arising from the Jade Kingdom, a realm in which all surviving otherworldly creatures found shelter, after humans began killing and hunting them. The Sentinels still protect the earth whenever Oni walk it. So they return through their jade portals. Possessed with the magic of ancient times, like the Kami had, they can hide in plain sight. Their jade armor hides them, when a mortal is looking at them.
As Oni appeared in several times during earth's history, the Sentinels adapted to these times. Although they are ancient warriors, some use modern technology while others stay traditional. But in the end, they are here to save the earth - but not the humans anymore. They walk between both worlds, the protectors of two realms.