Beyond the literal character of Roscuro, chiaroscuro—the artistic balance of darkness and light—is an enduring motif throughout the novel. The main characters continually move between the brightly lit upstairs and the dark dungeon. This is also a reflection of their flawed, realistic morality: Just like condemning rats to darkness and evil, and mice to light and goodness, is a simplistic categorization, nobody is ever all good or all bad. Every character harbors love and hate for different reasons—some personal and some grounded in societal structures.

The two, darkness and light, must remain in balance with each other. Roscuro tries to imprison the princess in the darkness forever, but this would lead to no end. Similarly, Roscuro had no place to go after the banquet (and subsequent anti-rat laws) other than the dungeon’s darkness; the darkness he once loathed provided great comfort, just as the sunlight can be inspiring. This duality is a metaphor for the ups and downs, the fair and unfair, the general luck and sorrow of life.

Most of the characters express a ridiculous, yet hopeful sense of desire. For example, Miggery Sow longs for her deceased mother; Pea shares this sentiment.