Marty never changes?

In all the Back to the Future movies, as well as the video games, everyone else's personalities seem to be affected by the timeline. Everyone except Marty. No matter what, he always seems to be the same. Even his brother and sister were different in the second movie. Of course, the whole time machine concept is highly illogical and not possible at all. But I guess this is one of those cases where its "just a movie".