The Light Novel series is the source material for the adaptation of the Anime and Manga series. The information and terminology on the article will be based on the source material rather than the adaptions.
Both he and his brother Bonifatius were qualified to become the next archduke, but his brother renounced his claim for the title since he had no desire to lead.
Relatively early in his reign Aub Ehrenfest suffered a severe illness that left him close to death. To prepate for the worst Bonifatius was trained to take up the mantle and taught even the secrets the only the Archduke is supposed to know, but to everyone's relief the Aub recovered.
He would later marry his cousin Veronica and the two had three children together. The first two were the girls Georgine and Constanze, the former of which was believed to inherit the title of Archduchess over her little sister. Once the aruchducal couple gave birth to their son Sylvester what seemed like a foregone conclusion turned back into a fierce competition. Despite Georgine seeming more skilled and motivated to many, Sylvester's greater mana capacity and gender led his parents to choose him as the successor, leaving Georgine devastated. Goergine had harassed and mistreated her brother since the very beginning, but upon losing the title it only became worse. At one point their parents started to genuily fear for Sylvester's savety and thus married Georgine off to Ahrensbach, even though that meant she would only be the third wife to the local Archduke.
Apart from the three children with his first wife, Aub Ehrenfest also had a child with a concubine. This son was Ferdinand. Veronica always had been abusive to anything whom she preceived as even the most remote threat to her and her direct family and harassed and abused even her half-siblings and their children relentlessly. Though few felt her fury and paranoia more than Ferdinand. When Aub Ehrenfest's health declined, Veronica's influence and power grew accordingly... as did her abuse, particularly of Ferdinand to the point where Sylvester adviced Ferdinand to enter the temple to escape her.
Aub Ehrenfest succumbed to his illness shortly after and was succeeded by Sylvester who is now the seventh Aub Ehrenfest.
Due to being a mere Blue Priest Apprentice at the time, Ferdinand was denied a seat of honour at his father's funeral and could only watch from a distance as a priest.