How about Robcina A for the headcanon thing?
Aftercare - What they’re like after sex.
(Going with NB!Robin here!)
So, these two both have some pretty deep PTSD that makes aftercare a necessity for them. Neither is used to being physically vulnerable, so they need downtime when they are done. Long nights of non-stop sex is unusual unless it is after a particularly dangerous battle or they have been apart for some reason.
Lucina is less experienced than Robin, so she can feel dizzy or bashful afterwards, either from being pleasantly overwhelmed or hoping she didn’t say anything stiff or unusual in the heat of the moment (she usually does, but Robin thinks it’s cute.)
Robin is a night owl, and Lucina gets restless staying inside for too long, so they like to take long walks under the moonlight after they’ve spent the night together.
They only talk about their sex life behind closed doors, so they simply walk hand in hand and talk about their plans for tomorrow and joke about the other members of the army.
(Lucina loves reflecting on these walks because she never thought she would get such simple, happy moments after everything that she’s been through)
They also like to spoon, and they switch a lot between big and little! (They secretly prefer Robin as the little spoon though, so Robin can read to Lucina and she can read over their shoulder)