The Leafé Knights are the protectors of Leafé, the source of life in the world.
There are seven Leafé Knights total. Each Knight has control over his own element. There's Hayate the Leafé Knight of Wind, who can make wind as sharp as blades and even create a sword made of wind. Sasame the Leafé Knight of Sound, who has amazing hearing and can make shock-waves out of sound. Go the Leafé Knight of Fire, who most of the time just throws fireballs. Kei the Leafé Knight of Light, who most of the time just throws balls of light. Mannen the Leafé Knight of Ice, who most of the time just freezes things to spite someone. Hajime the Leafé Knight of Water, who can create whips made out of water. And Shin the Leafé Knight of Wood/Plants, who (at least in the anime) can create containment fields to protect the world from getting damaged during a battle, and seal the evil Demon Larvae and Transport Demons.
Sixteen years ago, the seven Knights almost died trying to stop the Princess of Disaster. Mannen, Hajime, and Shin (adults then) did die, in order to stop her. The other Knights, had to heal themselves restoring what little Leafé they had in them. However, since their was no Prétear, there was no one to create more Leafé for Mannen, Hajime, and Shin. They had to be born again, and grow up like normal children, without any memory of their past lives.
- The seven knights are practically immortal. They live on the Leafé, and it gives them their elemental powers. That's why they look the same as they did sixteen years ago. However, they can die if there is no Leafé to sustain them.