Between Cinderella or Kafka's Cockroach.
This takes place during the Water Seven arc. Spoilers for the later parts of the arc. The title is taken from the 31 Days theme for September 22, 2006.
As she steps away into the alleys of Water Seven, as she prepares to commit yet another one of many betrayals against people who could have been friends and will definitely be enemies, Nico Robin remembers what it was like, to stand tall on the deck of the Go Merry in the wind and sea and feel something almost like contentment. It had been fun with the Straw Hats, fun in the sort of way that she had never allowed herself to have or feel for a very long time.
Out there, with Monkey D. Luffy's boisterous cheer, Usopp's incredible enthusiasm, Zoro's rough attitude, Sanji's easy flirtation, Nami's crooked smile and Chopper's gruff concern, it had almost been like home.
She is, however, afraid of admitting that to herself, or to anyone.
She has spent twenty years carrying the memory of Ohara in flames and her mother's eyes, and she doesn't intend to stop anytime soon. Not for them. Not for anyone.
Nico Robin is the monster who cannot feel, and the more she believes this, the easier each and every betrayal becomes.