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.
This feature is available after the “Out of the Closet” mission in Brucie's mission line.
Conversely though, Niko's choice of wardrobe and vehicular transport will also affect whether or not his date has a good time.
After a couple of successful outings, “try your luck” when dropping the date off and if she accepts you will later get a phone call from her, unlocking the special ability. Her online username is “Law Chick.” Kiki is quite easy to please; she prefers clothes from the Russian shop and downscale cars. Special Ability: Lower Wanted Level - Call Kiki to have her lower your current wanted level by three stars.
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.
Call your friend and perform activities with him on a regular basis to increase your 'like stat'.
You can perform upto four activities on each outing in order to give your 'like stat' the highest possible boost.
If your friend calls but you aren't able to hang out, respond positively and then call immediatley afterwards to cancel the activity. There are five different girls which Niko can date, two of which are unlocked automatically during the storyline, and the rest of which are available via the in-game internet.After a date, you need to wait 5 hours (10 real-time minutes) before calling your girlfriend again.