Gideon

 is Leo's former mentor.

On the Show
The sisters first met Gideon in season six after they were introduced to the existance of magic school. They learned that he was Leo's mentor when he was first a whitelighter.

The Darklight Zone
In a flashback Gideon is seen in front of the Manor with Leo telling him about his new charges.

Volume 4

 * "These new charges will be difficult, but now that Rennek is no longer a threat, the Elders feel that you are ready." - The Darklight Zone