When Marie is first met, she seems to be a very cold and antisocial individual. She is cranky, sarcastic, irritable, very foul-mouthed and often prone to mood swings. She will not hesitate to voice out her opinion or express her thoughts, regardless of how it would make everyone feel. As the player proceeds in her Social Link, however, it becomes apparent that her "lack of emotion" is basically just her way to deal with her own shyness, insecurity and naïveté. This is also why she wants the Protagonist to show her Inaba: She wants to learn about the world and who she really is.
Marie sometimes expresses her thoughts in poems which often deal with depressing themes like farewells and existentialism, questioning her own origin. Besides this, Marie is into fashion, loves nature (another major theme in her poems) and is apparently concerned about her figure, which is why she only eats healthy food.