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.
Rudiger appears to be a very polite person.
Upon the nobles from Frenbeltag introducing themselves to Rozemyne and Wilfried, Rozemyne notices hostility from them likely due to reducing her mana-support to their homeduchy upon assuming the rank of High Bishop, which resulted in the already straining duchy falling on even harder times, though Rudiger himself appears perfectly polite.
When Detlinde invites Wilfried to a tea party and outright rejects Rozemyne's attempts to join too on the argument that she too is her cousin, but Detlinde harshly rejects her and states that she does not see her as a relative since she is only adopted. At this Rudiger steps between them and asks if he may join to which Detlinde agrees after a pause. His motivation for just stepping between Rozemyne and Detline aren't clear. Since Detlinde was at the time raining ridicule on Rozemyne it is possible he stepped in to protect her, but he did so in a manner that can also be seen as rude and dismissive, especially considering he is from a duchy ranked lower than Ehrenfest.
At the tea-party, Rudiger carefully inquired about whether Rozemyne was already engaged with anyone, hinting that Frenbeltag might be seeking to arrange a marriage between the two for political and mana-related reasons, but Wilfried declined to give any information, telling him to leave such questions for the archduke's conference.
Later at a tea party where Ehrenfest invited all other duchies at the same time, Rudiger is one of the few people who make the effort to bend over a little to be closer to eye-level with Rozemyne, rather than looking down on her, which she greatly appreciates. He also mentions how he hopes to get along well with her since they are relatives and wishes for their respective duchies to keep their close alliance and friendship.
When Detlinde uses fake-pity to expose Rozemyne had grown up in the temple and is still fulfilling duties there in religious ceremonies to undermine her reputation, Rozemyne retorts after entering noble society Aub Ehrenfest himself asked her to keep providing mana to the temple due to the mana shortage and that she envies the greater duchies who can afford the luxuary of not having the archduke's family help provide mana for the duchy. At this she is backed up by Wilfried who states that he too provides mana for the temple and Rudiger joins in stating how hard the middle and lower duchies have it with the current mana shortage, soon to be joined by nobles from other lower-ranking duchies, making Detiline's attempts to besmirch Rozemyne backfire by providing her with sympathy from the other nobles instead.
At the fellowship gathering at the academy the following term, Rudiger is happy to reaffirm the friendship of Frenbeltag and Ehrenfest once again. He informs Rozemyne, Charlotte and Wilfried that their example of providing their mana to the Spring Prayer inspired him to ask his parents to be allowed to do the same. With their permission he joined their temple's efforts during the ritual and as a result Frenbeltag enjoyed a considerably better harvest, bringing hope back into the eyes of their dejected nobles.
- Ascendance of a Bookworm. Part 4 Volume 1. Chapter 10: Royalty and Nobles from Other Duchies
- Ascendance of a Bookworm. Part 4 Volume 1. Chapter 20: Dedication Whirl
- Ascendance of a Bookworm. Part 4 Volume 3. Chapter 7: Return to the Royal Academy
- Ascendance of a Bookworm. Part 4 Volume 3. Chapter 11: The Tea Party for All Duchies
- Ascendance of a Bookworm. Part 4 Volume 5. Chapter 18: The Second-Year Fellowship Gathering
- Ascendance of a Bookworm. Part 4 Volume 7. Chapter 17: The Graduation Ceremony