Edmund becomes more and more assertive throughout the book. In the beginning of the book, he is very timid and compliant. Edmund and his sister have been left in their room by their Aunty, who has gone away and not returned for two days. There is nothing left to eat in the house, and Edmund's sister, Sis, believes they need to take charge and look out for themselves. Edmund, though, is hesitant. Aunty has instructed them to stay put, no matter what, and he does not think they should disobey, under any circumstances. Edmund has great respect for authority and is used to doing "what he is told" by adults, to the point that he, unlike Sis, is afraid to think for himself.
In the middle of the story, Edmund, bewildered by Dupin's erratic behavior, begins to question whether adults are, indeed, always correct and worthy of unqualified obedience. Although he is still very compliant to directions given him by adults, he acts on his doubts on occasion, interrupting Dupin in mostly futile attempts to keep him on track. Edmund would never have imposed his own will on an adult previously, but his experience with Dupin and his desperation to find Sis leads him to assert himself at times.
By the end of the story, Edmund, although still timid, has enough gumption to openly defy Dupin, who, despite the fact that he is an adult, is clearly not in full possession of his faculties. He manipulates his conversations with the man in order to keep him active in the search for Sis, and even snatches Dupin's precious notebook and threatens to destroy it if Dupin does not focus on the task at hand and find his sister.