When the darkest part of you meets the darkest part of me, it creates light.
-- Frank (Chapter 2)

Importance: Frank tells Cleo this as a part of his vows during his wedding. It reveals Frank's fixation on his vices and the romanticization of each of their struggles, foreshadowing the destruction to come. When Frank's alcoholism meets Cleo's depression, the two problems have a malevolent effect on the other, perpetuating more drinking and more sadness, respectively. Frank's vows are ultimately disproven when the characters split up, and Cleo reflects on his vows. She transforms them by claiming that she has met the darkest part of herself, and out of this meeting comes creation in the form of her art. This reclaiming indicates she is healthier on her own than with Frank.

[Cleo's] marriage had given her access to a world he would never know. She would not admit it, perhaps would never consciously know it...
-- Quentin (chapter 3)