Vivian is thirteen the first time her father arranges meetings with some of the other nobles and kings with daughters her age, and that summer Princesses Elena and Mithian come to stay with her for a fortnight. It's all horribly awkward for about five days, Mithian two years older than them and Elena always staring longingly at the stables and falling over every third step, and then somehow they're all sitting around in Vivian's chambers one night after their fathers all sternly told them to go to bed and winked at each other where they thought their daughters couldn't see and suddenly Mithian and Elena's lent-out maids suddenly knew about the secret passage to Vivian's rooms and they went because obviously it was duty. Conversation is scant and awkward, but they're all dutiful daughters who love their fathers, if nothing else, so they make do until Mithian accidentally kicks a pillow into Elena's shoulder.
Elena's retaliation is swift and, judging by the shocked expression on her face, mostly instinctual, but Mithian seems to seize the opportunity to be over the awkwardness and enters into the fray with a cry, and Vivian joins in because if they are going to ruin her pillows there will be hell to pay and within minutes they've dissolved into a flurry of giggles and feathers and are talking with each other like they've been friends their whole lives.
Their fathers find them in the morning, all asleep in a ruined mess of feather pillows that Vivian's father won't even scold her for having to replace, and they all smile and call it a success. Vivian makes sure to write letters to Elena and Mithian after that and gets spiky handwriting and distracted observations from Elena and neat script and kindness from Mithian, and they all three don't see each other often, but they do some, and they always spend at least one night per visit on the bed of whoever's hosting them, giggling late into the night.
(There's another sleepover, years later, when they've all three been rejected by the same King and Vivian can't seem to stop herself talking or thinking about him. Elena's gained some grace, almost a different girl if it weren't for the same smile, and Mithian's just the same as ever, even though it's been a few years since they were all in the same place at once, probably since before Vivian went to Camelot. Still, they enjoy each other's company just as much, and the night they spend in Vivian's room, Elena interrupts her swooning over Arthur to turn to Mithian and say "You're right, it's like in a fairy tale, we ought to try it" and Mithian smiles and puts a hand on Vivian's knee and says "Which one of us should go first?")