Each friend offers a 'special ability' in return for maintaining a good friendship.
Denise is first encountered during the mission Burning Desire for Frank Tenpenny and Millie first appears during the mission Key To Her Heart in Las Venturas.
The remaining four girlfriends are Michelle Cannes, Barbara Schternvart, Helena Wankstein and Katie Zhan.
However, that shouldn't daunt you from building as many relationships as you can, because they happen to provide some of the deeper and more rewarding experiences Grand Theft Auto IV has to offer.
In fact, without building up relationships with myriad characters in the game, you're going to miss out on a lot of what the game intends for you to see.
These relationships take on a more light-hearted and pseudo-romantic tone and are completely optional for the most part.
There are two females in the game you must date—Mallorie's 'friend' Michelle and the O'Reary boys' sister Kate—but other than that, dating is completely optional.
Pursuing the Girl Taking the Girl Home Community Q&A Grand Theft Auto V is an action-adventure video game in which your character is allowed to roam free and partake in a myriad of activities.
Although you're not able to get a 'typical' girlfriend in the game, you can have a girlfriend whom you are able to contact whenever you want.
Denise is first encountered during the mission Burning Desire, whilst Millie first appears during Key To Her Heart.
The other girlfriends, who are encountered during gameplay rather than being attached to missions, are Michelle Cannes, Barbara Schternvart, Helena Wankstein and Katie Zhan.
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).