According to Eric Goldman of IGN:
"But in Season 5, we’re going to finally see at least one Republic Commando in action. This gentleman’s name is Gregor, and as the clip shows, he’s got a group of droids he’s working in tandem with, including R2-D2 himself. As we learned at The Clone Wars panel, Artoo and these droids are tasked with infiltrating the separatists to get a crucial encryption code, and along the way, they meet up with Gregor. While the specifics on his history and involvement in the storyline have yet to be revealed, you can see in the following clip that his skills in battle become crucial to this mission's success."
The episode(s) that Gregor appear in are ones that are written by TCW writer Brent Friedman as he confirmed via Twitter:
These episode(s) appear to be the R2-D2 centered story that has been mentioned in Star Wars Insider and that we see in the extended Season Five trailer. It makes a ton of sense to have a human character to play off of the droids for dramatic purposes.
The IGN Republic Commando Trailer:
SOURCES: IGN and Twitter