A Guardian is a Light Fae who devotes his or her life to serving a higher purpose.
The Guardians, in servitude, donate their body for the protection of something precious and crucial to the Light Fae. They all have a tattoo of a special symbol on one hand that identifies who they are.
A Guardian, who was also a Doonie, stopped to help in what he thought was a roadside emergency, but was instead killed and flayed by Sluaghs in order to procure a secret map that was tattooed on the inside skin of one of his arms, which led to the location of The Heart Stone .
The corpse of a Guardian called Ralph was found in a dumpster with an organic flexible tube implanted along his spinal column. When Lauren used radiology for a closer inspection of his body, it revealed that the tube was actually the sheath of a sword — the Sword of Agros. A Sluagh had killed him to pull the sword out and used it to break The Heart Stone.
Their willingness to donate their body in service to their clan indicates that those who serve as Guardians are benevolent Fae.