He is Barkhorn. He was first introduced in a door in 3rd, as the 8th Dominion of the Gralsritter. His codename is "Roaring Lion. It was he who wrote up the report on the Salt Pale that struck North Ambria. In CS1, we find out that he was the wandering priest who would travel to Gaius' village and minister to them.
In CS2, right before you head out to rescue the royal family, you can take on a quest from the priest in Trista, and you basically end up flying around Eastern Zemuria in the Courageous, tailing Barkhorn in his faster (and cloaked) Merkabah. He knew that you were following him and gave you some of his quartz as a gift.
In between CS2 and CS3, a border skirmish between Erebonia and Calvard breaks out in Nord. Calvardian airships attack Gaius' village encampment. Barkhorn bodyblocks airship gunfire for Gaius, and transfers his stigma to Gaius. According to Gaius, Barkhorn then died from the blood loss. With Gaius having inherited Barkhorn's stigma, Gaius became the new 8th Dominion.
It would appear that Barkhorn did not die, but instead used the opportunity to fake his death. It is possible that he really did die in Gaius' arms and then was secretly resurrected. We know from 3rd that Kevin had the Salt Pale weaponized into a Salt Bolt explicitly because he feared that Weissman could be resurrected, which implies that there are other means of resurrection outside of what we see in the Erebonia arc. Alternatively, Gaius could have been lying to his friends that Barkhorn died in order to cover for him, but that would be inconsistent with the tone with which Gaius retells the story, his characterization, and the tone of the Trails series. In CS3, we find out that the SSS told Juna - a random SSS fangirl - about Wazy's true identity, so it's not like Trails protagonists care about keeping the Gralsritter's identities a secret.
The real question is whether or not Barkhorn is 1. do the Gralsritter know that Barkhorn is alive? 2. is he still working for the Church? And 3. is he the 5th Anguis, who has inside intel on the Gralsritter?