I would argue that one of the most important minor characters in this brilliant novel is Anna, the elder sister of Oskar's grandmother, who dies during the Dresden firebombing and leaves both her lover, Thomas, and Oskar's grandmother bereft. Why she is so significant is that she is the unspoken shadow that prevents Oskar's grandparents from having a truly happy marriage. Thomas only marries Oskar's grandmother because she is but a pale imitation of Anna, and she is the unspoken ghost that haunts their marriage and prevents them from achieving happiness. Note how Thomas constantly looks for Anna, and he does this in the "life" that Oskar's grandmother writes of her own experiences:
I was looking for Anna, too, I searched and searched, I got a paper cut on my forefinger and bled a little flower onto the page on which I should have seen her kissing somebody, but this was all I saw:
Anna then is an important minor character because even though she is absent she is very much present as a burden in the lives of Oskar's grandparents and as something that prevents them from achieving happiness.