Miss Nancy is the CEO of Tomorrow Corporation. She is one of three characters that send you letters while playing the game. These letters will contain information about your game progress, Little Inferno and achievements.
She will send you the catalog after burning the Terms and Conditions (which is also sent to you after burning the Instructions that are already in the fireplace when you start a new profile.)
At one point of the game, she will send you a free hug coupon, which you may choose to keep or burn. Either choices will affect a small part of the ending. If you choose to burn it, she will tell you that she hopes it kept you warm. If you chose to keep it, she will give you a great big hug. These choices do not affect the ending overall.
Upon completing all 99 Combos she will send you a mousepad.
Her catchphrase, in which she usually ends most of her letters is "Mmmm Mmmmm MMMMM weeeEEEE!" followed by "Love, Miss Nancy."