Friends and girlfriends become available as you progress through the storyline.There are five friends and five girlfriends in total, and each character has a unique set of preferences.Don't take your friend to an activity that he doesn't like or your 'like stat' will reduce.You can perform upto four activities on each outing in order to give your 'like stat' the highest possible boost.Aside from the amusing interactions between Niko Bellic and the women he dates, there can be several in-game benefits to starting a relationship with any of the datable characters.Putting in a little bit of time to raise the fondness of the girlfriends can unlock their "special abilities": these can help in particularly difficult missions late in the game.
After the Brucie mission "Out of the Closet", Nico can start to set up dates through the in game internet dating site and Her profile on the LCPD Database also states that she is a nymphomaniac.Carmen usually refers to herself in third person, e.g. She also considers herself very attractive, and upon entering strip clubs will say, "My tit job is so much better than the ones these girls have got." She also sometimes has her nails and hair done.(However, wearing the wrong clothing or having the wrong car doesn't actually result in a date failing, only that the girlfriend makes negative (or snide) comments.) Assuming no failed dates, Niko can usually get to enjoy "coffee" (i.e.a sexual encounter) with the girlfriend at the conclusion of the second date, with the exception of Kate, who will never allow the relationship to reach that point no matter how many times she is dated.