serah is snows fiancee, but when shes cursed, snow is trying to save her even when shes become crystal (thats the curse which equals pretty much death). shes stubborn, kind, sweet and also very compassionate. lightning is her big sister who didnt approve of their marriage plans and didnt believe her when she told her about the curse (which is so ancient that nobody would believe..) and that really hurt her since she loves her sis a lot. snow is also very stubborn, very loyal, always trying to save everyone, but in reality hes also insecure in what hes doing, BUT hes still doing it because he genuinely wants to be a hero and the people he loves to be save. hes her hero and a good guy while serah also is a good girl - they fell in love in summer vacation :’) he gave her a necklace that looks like their world because he wanted to give her the world.
the story is really big and theres so much more to their characters than i could possibly tell you in a few sentences XD maybe you could draw them in wedding clothes because we never got to see their wedding (because she dies in part two.. again.. ;_;)
heres a cutscene of them after he proposed to her: http://www.youtube.com/watch?v=XmTbPp78XAU
heres some reference to their clothes/appearance
(his design in lightning returns looks so good! here http://static2.wikia.nocookie.net/__cb20130606183131/finalfantasy/images/8/8c/LRFFXIII_Snow_Villiers.png )