Even at Garreg Mach, Dedue couldn't be at Dimitri's side all hours of the day and night. That had never been a feasible arrangement, even when they were children, nor could it ever be one. He will always wish to serve His Highness as his sword and shield-- and yes, Felix, he heard you the first time when you said that you are now the Shield of Faerghus-- but no man carries his sword and shield with him at all times.

And as far as Felix becoming nothing to Dimitri, well. It isn't a possibility that Dedue had ever even entertained for more than perhaps a moment. Felix is, regardless of whether Dedue thinks that he's a palatable person or not, one of Dimitri's childhood friends, and a relationship that he had dearly wanted to maintain. For Felix to leave his life entirely would surely pain him, and Dedue wouldn't want anything that causes Dimitri pain.

So, he has no reason to try to drive Felix out of His Highness' life. As abrasive as he finds him, he will simply have to live with his presence, as he lived with it while they were living at the monastery. At least if he's reconciled with Dimitri, Dedue oughtn't have to worry about fighting the insistent urge to toss him out of a window for calling His Highness a boar prince.

That isn't to say that he might not still entertain the thought, but only privately and with no intent on following through.

"I see," he says. He can't find fault with Felix's decision-- wanting to be at Dimitri's side for love of him rather than just the demands of duty. "Then I am glad that you have reconciled. Your harsh words had always troubled him."

An understatement, really.

"Since you have said that you are from a later time than I," he says, "how long ago was Gronder Field for you?"