Though not having officially debuted in the main story, Berstetz plays a rather important role in Ex4, as Vincent's second in command, immediately springing to action in order to indentify the culprit behind the coup d'état.
Berstetz is a very old man of short stature. He has relatively long, tied back grey hair and an aged and wrinkled face. Underneath his slightly crooked nose, he sports a bushy mustache. Despite his position as the Imperial Prime Minister, Berstetz can be seen wearing a rather humble and every-day, long-sleeved attire.
Berstetz is a very wise and shrewd old man who's is easily able to outsmart people. He also appears to be very loyal to the Emperor, working behind the stage in order to eliminate any and every threat that's facing Vincent, as well as leading the Empire when Vincent isn't able to, as seen during the events of Ex4.
Having said that, Berstetz does seem to have more going for him than what meets the eye as Vincent himself seemed to be aware that something's off with the old man.
During the Imperial Selection Ceremony, Berstetz acted under Lamia Godwin's command, helping her achieve her goal of slaughtering every single one of her half brothers and half sisters in order to be crowned as the 77th Empress of the Vollachian Empire. During the battle at the Abellux Manor, Berstetz helped to command the army led by Lamia, commanding the artillery on her behalf. After Lamia realised the battle is lost, she ordered him to cover her retreat. Following Lamia's death and Vincent's coronation as the 77th Emperor of the Vollachian Empire, Berstetz started working under him and was, at one point, appointed as the Imperial Prime Minister which granted him political strength second only to the Emperor himself.
Half a year before the events of the first Arc, two members of the Sage Council, Miklotov McMahon and Bordeaux Zellgef, accompanied by the Sword Saint, the Finest of Knights and the Blue, went on a diplomatic mission to the Empire, in order to sign a non-aggression pact between the Dragon Kingdom and the Empire. While not immediately meeting with the five diplomats, Berstetz acted in the shadows, swiftly getting rid of the traitorous noble who planned the coup d'état during the aforementioned diplomats' visit in the Empire, effectively keeping the overthrow at bay.
- "Hmph. And to think, I very nearly had the tiger by the tail" Re:Zero EX Light Novel Volume 4: Diplomacy by Bloodshed. Section 19.