A person's ethos is their characteristic spirit, as displayed in their beliefs and actions. Christopher Boone, the narrator of The Curious Incident of the Dog in the Night-Time, is a curious mixture of logic and randomness. Perhaps this might be said of anyone—we are all rational in some matters and irrational in others—but Christopher is an extreme example. He loves numbers and is an extremely gifted mathematician. One of the first things he tells the reader about himself is that he knows all the prime numbers up to 7,057, and he is so interested in prime numbers that he uses them as chapter headings. However, he has an almost pathological aversion to the color yellow. This surprises the school psychologist, who points out that Christopher is very logical in other areas, but this prejudice seems illogical.

Aside from his love of logic and numbers and his very particular aversions (which are only unusual in being more definite and explicit than those of other people), some of the strongest elements of Christopher's ethos are his love of calm and solitude, his delight in knowledge, and his dislike of excessive amounts of information and physical closeness to other people. Most of all, perhaps, Christopher is distinguished by his extreme honesty. He dislikes metaphors because metaphors are lies, and he punishes his father very severely upon finding out that he has been dishonest. Christopher's truthfulness and literal-mindedness are at the core of his ethos and inform everything he says and does.