According to Hideo Kojima & Yoji Shinkawa in the documentary on the making of Metal Gear Solid 2 from the game's bonus disc, Shinkawa stated that Rosemary represents Kojima's ideal, classy and intellectual woman. Some of Raiden & Rosemary's conversation were influenced by Kojima's own life experience.
Kojima stated that the idea for April 30th stemmed from his relationship with his wife, where she kept reminding him about that date before they were married. One evening, he had a date with his wife, but he completely forgotten about it, which caused him a lot of crisis in their relationship.
Contributed by ProtoSnake