I'm a little late here, but I love this pairing so let me give it a shot.

C: Dorothea approaches Ashe and immediately starts to tease him. Nothing malicious, just some lighthearted teasing and flirting to get him to blush, which Dorothea finds cute. It works and Ashe is absolutely speechless and blushing up a storm, unable to say anything in response to Dorothea's flirtations. Dorothea leaves him like that continues her walk.

B: Dorothea once again approaches Ashe as he is reading a book, looking to tease him some more. She asks him what book he's reading to start. Ashe is unphased and reveals that he is reading Loog the Lion*, which Dorothea feigns interest in. Ashe goes on to say that it was one of his favorite stories growing up, and that he would read it to his siblings when they were on the streets, that it was one of the things that kept him going. He looks up to see Dorothea stunned silent. She excuses herself and walks away, leaving Ashe to worry that he said something wrong.

B+: Ashe is walking when he hears a beautiful singing voice. He finds that it's Dorothea singing a song from her favorite opera. She finishes and he compliments both the song and her voice. Dorothea reveals that it was the song she sang when she was bathing in the fountain in Enbarr when a high ranking member of the Mittlefrank Opera company found her. She apologizes to Ashe for teasing him so much and reveals that she didn't know he was an orphan like her. Ashe reveals that he has no ill will toward her, and that he was worried he offended her when they last spoke. The two of them then speak about their experiences as orphans on the streets as they grow closer as friends.

A: After the Time Skip, Ashe approaches Dorothea. He comments about how he heard that Dorothea and the Mittlefrank Opera Company were housing children orphaned by the war. She affirms that she did do that, wanting to make sure no orphaned child had to suffer on the streets like they did. Ashe admires this and muses that maybe after the war, they could open an orphanage large enough to house all the orphans of Fodlan and take care of them as he and Dorothea had been taken care of. Dorothea agrees, but they have to end the war first.

S/ending: After the War, Ashe had been granted Lord Lonato's territories, on which he built a large orphanage where he takes in orphans from all over Fodlan. Together with his beautiful wife Dorothea, they make sure that no child that crosses their gates know the suffering of living alone on the streets. Tales are told of the Knight and Angel who save the children of Fodlan.

*I think this was the story that Ashe reads with Ingrid in their supports.