This stat (amongst other friend stats) can be accessed from the pause menu under the 'Stats' section.
Since Kate and Michelle are fairly easy to please and only pertain to the storyline, they won't be covered here.
There are three other girls that Niko can meet and date via the
As you progress through the story, Niko will be able to wine and dine with a number of Liberty City women.
Dating is much like hanging out with friends in that each girl has their own preferable activities.
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.