In Act 3 of the event, the two of them take a drink at the Angel's Share and Kaeya was about to finish his drink when the Traveler appears. At one point, Venti asks where is the bartender because Diluc is here. Rosaria told him that Charles is out on delivery, and Kaeya jokes to her by saying "Hahahaha, and who is most directly responsible for creating the demand for an alcohol-delivery service at this time of the year in the first place?", implying that Rosaria takes a lot of drinks. Then Venti asks about the mixing ability of Diluc, and again Kaeya teases Rosaria by saying "Why don't we ask the formidable Sister Rosaria... What do you think?". Subsequently, she sigh a bit and respond that he's not bad. Afterward Paimon asks if they had a fight. Kaeya said that he calls it a misunderstanding. He caught sight of a suspicious figure hurtling by the Knights of Favonius headquarters, but fortunately he discovered in a good time that it was "Miss Rosaria", as he calls her, and she stayed in his hand. After having discussed a bit, they all toast to the festival and the free people of Mondstadt.
After Act 4, in a cutscene where a lot of characters of Mondstadt are shown, we see Kaeya and Rosaria always at the tavern and they are discussing. Kaeya talks to Rosaria by saying "Of course, any good tavern offers bar snacks" with a drink in his hand. Rosaria replies "Oh really?" the hand holding her head looking at Kaeya, while Diluc took the bottle out of their reach.
At the tavern, they often have a discussion over a drink and discuss about their secrets with each other.
In Rosaria's voiceline about Kaeya, she says that wine has its uses, confirming their discussion over a drink, like revealing someone's true colors. To her, "Smooth talk is Kaeya's veil, but being me, I can see right through it." She also jokes by saying that we should see him when he's drunk.
In Kaeya's voiceline about Rosaria, he says that he cannot tell us about her secrets, but it seems that he knows them. Kaeya also says that his impression of her is not bad, and describes her by saying that "She's honest with herself. She casts off that which she has no time for and she doesn't feign motivation for affairs that do not interest her."
- “Ahaha, you want to know about the secrets of Sister Rosaria? That... I cannot tell you. Though my impression of her is not bad — certainly better than a certain someone whose name I won't bother to mention. She's honest with herself. She casts off that which she has no time for and she doesn't feign motivation for affairs that do not interest her. ”
- — Kaeya's voiceline about Rosaria
- “Wine has its uses... like revealing someone's true colors, for example. Smooth talk is Kaeya's veil, but being me, I can see right through it. Hah, you should see him when he's drunk. ”
- — Rosaria's voiceline about kaeya
Fans began shipping Kaeya and Rosaria after the 1.4 trailer showed them drinking together, and then the event. Based on this and Rosaria's voiceline about Kaeya they are speculated to be frequent drinking buddies. Rosaeya shippers enjoy their dynamic as two characters who shroud themselves in mystery but let loose around each other. They are a rarepair who have mostly shipped as either platonic drunk buddies or a semi-comic love triangle with Diluc.