XIao-Dan is the universal little spoon omfg
Xiao-Dan and Kasamatsu would probably switch off or cook in tandem. I don't know why but I can see him being the sort to not want her to overwork herself and would help around ; v ;
How often do they fight?:
I doubt either of them would really want to fight, what with how the two of them are so shy around each other.
What do they do when they’re away from each other?:
Like always Kasamatsu would be a good captain for his team while Xiao-Dan continues to practice on her guzheng, but whenever they had time (since they're away from each other a lot because of their schools) they would text each other or call at night. The conversations would be short, but they would be nice~
Nicknames for each other?:
Kasamatsu to Xiao-Dan; Probably Wenhe-chan, and if the time ever came to where they started using first names for each to her, then Xiao-chan > w <
Xiao-Dan to Kasamatsu; Kasamacchi, since she uses "cchi" to people she feels really close to, but once they start going by first-name basis it would probably turn to Yukkun ; v ;
Who is more likely to pay for dinner?:
Knowing the both of them they would probably come to have an agreement that even when dating they would split the bill 50/50
Who steals the covers at night?:
Probably Kasamatsu would, on accident, but I can see him being a light sleeper so once he does he probably would wake up and then spoon with Xiao-Dan with the blanket over both of them.
What would they get each other for gifts?:
Since he's bad with girls Kasamatsu would probably ask Kise about what to get and end up giving her things like stuffed toys and flowers.
Xiao-Dan, knowing that Kise is good friends with Kasamatsu, would ask what to get him and probably end up getting whatever she thought would be nice for the occasion; chocolates on Valentine's Day, something he's always wanted for Christmas or his birthday, things like that. At some point though she would give him something homemade; either homemade chocolate, a cake, or a scarf for the winter.
Kasamatsu would remember the big things
Xiao-Dan would remember the little things
It's a symbiotic relationship, really.
Kasamatsu, but probably under his breath and when he's not thinking; once he realizes he would most likely go off on a tangent apologizing and getting flustered while Xiao-Dan tried to comfort him and smile and say that it was okay.
What would they do if the other one was hurt?:
Since I know that Kasamatsu is a man who is not afraid to cry he would probably get really mad at himself for not protecting her well enough and give an "I'm sorry" as he's holding her hand or holding her close. Then after he would leave he would probably cry in the privacy of his home/room and be just really mad at himself ; ^ ;
Xiao-Dan would be holding onto Kasamatsu's hand or hugging him really securely and would openly be crying. He would wipe a way her tears and say that it's alright but she would absolutely be hating herself for not being able to be there to keep him safe or to protect him.
After some trial and error Kasamatsu's lips finally found Xiao-Dan's ey.
Who made the first move?:
{I had to use a name picker for this omfg ;;;;;;;;;}Kasamatsu would make the first move~
Who started the relationship?:
I think Kasamatsu would start it first, since as awkward as he is around girls he's certainly less timid than Xiao-Dan is around generally everyone. So after a long time of them being awkward together as friends, now they can be awkward together as a couple.