Who is the Grand Inquisitor?

Question

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?

Answer 1

In "Shroud of Darkness" we learn not only that the Grand Inquisitor was a Jedi, as you have pointed out, but furthermore that he was a Temple Guard at the Jedi Temple on Coruscant.

That is, however, the extent of what is confirmed in canon. As indicated in the entry concerning the Grand Inquisitor in the official StarWars.com databank:

Not much is known about the Empire’s Grand Inquisitor. A Pau’an male, he was tasked by Darth Vader to hunt down any surviving Jedi Knights. The Inquistor was an intimidating figure, dressed in all black and wielding a red, double-bladed lightsaber. After several encounters with Kanan Jarrus and his Padawan Ezra Bridger, the Inquisitor died in a lightsaber duel with Kanan — but warned the rogue Jedi that his victory had unleashed something terrible.

(Source)

In particular, we know nothing of his record during the Clone Wars.

One further thing to add is that the credits for "Shroud of Darkness" refer to the Inquisitor as Sentinel, although it is unclear if this is a proper name or simply a description.