Vianne and Isabelle serve as foils for each other through much of the novel, and the narrative is shaped by their profound differences in personality and belief. When the two sisters are left under the care of an unkind Madame following their mother’s death, Vianne keeps her head down and urges Isabelle to do the same. Headstrong Isabelle, however, is not so easily able to repress her feelings, and she gets in trouble throughout her early years, first with the strict Madame who punishes her, and later at the various boarding schools where she rebels against rules and restrictions that she considers unfair and limiting. Though she is bold and brave from an early age, she is still quite immature in the early chapters of the novel, rushing into her activities with the Resistance without seriously reflecting on the risks involved. However, as she grows through her experiences of the war, she becomes an unstoppable force, coordinating an extensive system of contacts, safe houses, and transportation routes to help stranded allied airmen reach the consulate in independent Spain.
Though she dedicates her life during these years to serving the Resistance with extraordinary courage, she is nevertheless keenly aware of all the ordinary experiences that she has missed out on. After she is almost killed in Carriveau while hiding an injured American pilot, she realizes that time is precious and that she cannot wait to experience love, as her future is not guaranteed. The brief but happy time she spends with Gaëtan in the safe house in the Free Zone reminds her of what she has sacrificed. With some regrets, she resumes her work and, in the end, pays the ultimate price for her participation in the war effort. In the final days of her life, she feels at peace with her decisions despite all that the war has taken from her.