I just watched the Star Wars Rebels episode "Shroud of Darkness". In it, Kanan, Ezra, and Ashoka go to the Jedi temple on Lothal and meditate so that they can contact Yoda.
During Kanan's vision, he fights three temple guards. When he finally realizes that he has to submit and accept his fate, the lead guard dubs him a Knight of the Jedi Order and then reveals that he is the Grand Inquisitor and that he was once a Jedi himself.
Is there any canonical proof of who the Grand Inquisitor was before he was the Grand Inquisitor and, if so, did he serve in the Clone Wars?