Each character has missions to give you.
Completing one mission for them makes them your acquaintance, and you will have their contact on your messenger. (For Sparky this is Go to the cafeteria and find Sparky.)
Completing two makes them your friend, and you can ask them to follow you around.
Completing all (differs by character) enables you to propose to them and make them your boy/girlfriend. (For Sparky, it is enabled after n - 1 missions, and proposing triggers the last one, Get Sparky out of the closet, which you’ll have to complete before she could be your actual girlfriend.)