The difference in Kezia's character from her two sisters focuses on the way that she, unlike her older two sisters, has yet to learn of the sharp social divisions that divide her society from people that she should talk to and people that she shouldn't. If we examine the story, the major theme is that of social class and how it creates demarcated lines that cannot be transgressed in society. Clearly, in this story, such a line is drawn between the Kelveys and the rest of the children. However, what is different about Kezia is that she does not act as if there is a line that divides them. This can be seen when she invites the Kelveys in to see the house.
A symbol that is linked to Kezia's character from the very beginning of the story is the lamp in the doll's house:
But what Kezia liked more than anything, what she liked frightfully, was the lamp. It stood in the middle of the dining-room table, an exquisite little amber lamp with a white globe.
The symbolic significance of the lamp is made clear when this is Else mentions the lamp to her sister at the end of the story. It represents the warmth of human kindness that transcends man-made differences such as class. Kezia is different because she has not learned to act on such differences yet.