I’m wondering whether I should consider northwestern as a reach school. I do like the interdisciplinary education (I see myself as both a STEM and humanities kind of person), location, the moderate levels of stress (depending on what you do), and general freedom you seem to get from the university. Still, I heard that Greek life is dominant there and even people who aren’t in greek life seem pretty outgoing. Would an introvert like me fit in that school?
I am the father of 2 introverted NU engineering alums. Neither kid joined a frat or sorority - but they did join club sports, so that became their group outside of dorm and classes. Our D still flies to East Coast every year to party with her dorm friends, and our son’s ultimate frisbee skills will let him meet up with people where ever he goes. Even if he is still on the quiet side!
Good Luck with your decision! 
There’s definitely a large chunk of introverts here and I wouldn’t worry about that, but I will say that it’s pretty hard to stand out sometimes when you’re not at least an ambivert. That really doesn’t matter unless you’re optimizing for leadership positions on campus or whatever.
I actually ended up becoming more outgoing since arriving at Northwestern, however.