Uhm...'ser' is from the novels and TV show 'The Game of Thrones.' Outside of those, it's meaningless.
'Sir' is a title denoting knighthood. You're welcome to use it role playing for men and 'Lady' for women.
Given that you're talking about role playing, I recommend adding flourish to your style - go with 'Most noble Sir, welcome!' and 'Fairest Lady, I bid thee welcome!' etc.