The relationship aspect of Grand Theft Auto IV is perhaps one of the most nebulous areas of this epic game.
There are five friends in total, which become available at different times during the storyline.
If you want to date girls other than Michelle and Kate, you can do that, but it involves some legwork.
You're going to have to go to an internet café and browse the in-game websites Love Meet and Craplist (which are always on your homepage when you logon).
Each friend offers a 'special ability' in return for maintaining a good friendship.
In order to unlock a friend's special ability, you must have a high 'like stat' for that friend, and this stat is based on how regularly you hang out with your friend.
Dating these girls is not a requirement to attain the 100% game completion stat, however each has a special ability that is unlocked once they like you enough.