What are the girlfriend quests I need to complete in order to achieve 100% completion?

Solution 1:

I don't have a specific answer, but there are several "100% checklist" web pages that might be helpful, such as this one.

I can't see any girlfriend-specific tasks in that list though. This girlfriend guide does have some hints (see section 6):

Michelle and Kate are met through the game storyline and while dating them don't provide any real tangible benefit (Hot Coffee not withstanding) they do contribute to a 100% game completion

IMPORTANT NOTE If you want to get the perfect 100% girlfriend fondness that's needed for the XBOX360 achievement, you need to have Michelle's fondness at 100% before the mission "Snow Storm".

I have no idea if the requirement for the achievement is the same as the one for 100% completion in the game though...