Makoto and Shinji. They live in a small apartment in Tokyo. She teaches Japanese as a free-lancer. He was trying to become an actor, now has a regular job of some sort. They have been living together for two years. Shinji wants to marry; Makoto wants to keep things as they are.
Takumi and Keiko. For our purposes they are the happy couple that gets married, bringing up the question of whether Makoto and Shinji should do the same.
Towards the end of the movie. Makoto is sitting on the toilette, grieving. Shinji has been dead for a while. "Don't leave me" is how she feels but she knows he is dead.