They are friends since their time as a young officers in War College. They share information, friendly talks and respect each other's opinions. They often appear together and hailed as the pillar of Imperial Army. In Deus lo Vult upon deciding full assault on Legadonia for trespassing Norden, both of them opposed the notion as they foresee foreign intervention that could spell doom for their country.
Zettour have absolute trust in Rudersdorf, going as far as lending the man his precious treasure the 203rd Mage Battalion.
Zettour viewed Tanya as talented youth and his interest was piqued for her since selection review of War College. She went beyond his expectation with her foresight of global war, leaving him awed and terrified. In spite of his reluctance, Zettour decided to delve to insanity of placing a little girl under his command.
He likes her undeniably unexpected outcomes and ideas for the benefits of the country. He accommodates her as best as he could, especially during organization of her rapid response force. Zettour also didn't hesitate using her Thesis as the reference if something big crisis coming up e.g, during the Battle of Arlene where he used one of her Thesis as an advantage to prevent breaking some rules in accordance of International Law to win the battle.
Zettour notes that, if Tanya was a male, he would have happily allowed her to be betrothed with his unmarried granddaughter. He was also considerate to assign a female adjustant, Visha she is familiar with. And also posting a female doctor in 203rd's base.