Flirting pre timeskip
Dimitri is soft and courteous, inviting Byleth to solitary meals that someone else finds their way into. He compliments her whenever possible, especially on her smile
Dedue doesn't know how to skirt around his feelings and instead prefers to find her in the garden where he can see her nurturing side as well as casually relate her to the flowers without making her uncomfortable
Ashe is around every turn and corner waiting for any request he can help her with. He always runs her reports double time and when she's complimented him, red faced and panting, his face lit up like the night sky. He likes to tell her she reminds him of courageous heroes when they walk together, his goal being to force a bashful blush into her cheeks
Felix can't flirt and proclaims no interest in love. However he finds himself fighting harder around you. On a few occasions he even found himself challenging a few lurking students with red envelopes to a spare, running them off from her classroom
Slyvain is a master at flirting, he finds the words to express exactly how her wild demeanor made him feel. He expressed to her constantly the way she chipped away at his heart, only to be met each time with a curt nod or an annoyed sigh
Mercedes found constant ways to compliment your ability and figure, drowning you in dresses and sweets for every minor accomplishment. Her favorite was when Byleth would invite her to tea, the two would eat some of Mercedes' special sweets made with extra love and she would convince her to try on a few of her dresses. Mercedes' heart swelled to no end when the two ended up being the same size, and she commented on every graceful movement the smaller woman displayed
Flirting after timeskip
Dimitri now feared losing her more than anything, he was constantly by her side smirking at every cute face and stomach growl. He would laugh and wrap her in his cloak, leading her away to the dining hall. On one occasion, when they had figured out a particularly clever ambush, her blinding smile forced him to sweep her into a passionate kiss in front of the war council
Dedue knew exactly where she went when she was thinking and would join her in pruning the garden. When he was feeling especially affectionate he would wrap his hand over hers and put the other around her waist, sometimes sitting her in his lap as she worked on the shorter plants
Ashe began waking up a few hours before Byleth, having completed the most menial of her tasks before she woke up leaving her none the wiser for he knew she would protest. He would slip secretly back in beside her and kiss her awake each morning, telling her everytime how she was his goddess, a wonderful woman he would be worthy of
Felix didn't like it when Byleth strayed far, especially during diplomatic visits. Most of the noble snobs wrote him off as no real threat and would pursue his intended right in front of his very eyes. He would snake his arm around her hip and whisper into her ear naughty secrets he had promised not to repeat. He also began complimenting her while they would spar, noticing enough attention about the right thing would lower her guard completely for a moment as she stared off with a painfully adorable look. He would take his advantage and claim his prize
Slyvain would pull her into his chest and bend low to her ear to tell her she was beautiful, or clever, and how irresistible she was to him. He would pledge his love to her every morning and on the rare occasion they would fight he would propose to her all over again, kissing away tears until she was in his arms again
Mercedes was now confident in herself and her affection. Every now and then when Byleth was deep in thought she would cup her breasts and give them a soft squeeze. She would pepper her face in kisses when she was concerned and bring her heart shaped treats