If you want to date girls other than Michelle and Kate, you can do that, but it involves some legwork.
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Try to press the various girls on the site for dates, but keep in mind that there are some girls that you can date and others that won't date you no matter how hard you try.
So now that you know how you can potentially score a date with one of Liberty City's beautiful single females, what should you do when you're on a date?
And since you're almost certainly going to pick up a date in a stolen car, why not make sure that car is as fancy and well-taken care of as possible? Other than subjecting your date to vehicular homicide or in some other way murdering and dismembering her, few things will ruin the vibe of a good date from the very get-go more than being late to pick up your special lady friend.
Would it be corny to quote Good Charlotte's 'girls don't like boys, girls like cars and money' here? When a date is made, therefore, you want to be on time.
I've found on both the Xbox and PC version that Kate likes Modo. Before asking her out on a date, I make sure I change my Modo outfit completely from the last date with her. When you get to the point in the game where you are asked to make a decision over a MAJOR decision in Niko's life, my advice is - Save your game because whatever decision you make (don't want to spoil it) the outcome will surprise you - and there are two possible outcomes which come out of that key decision, both of which led to two different endings for the GTA-IV game.