The Devil symbolizes weakness, materialism, incomplete knowledge, and, in general, a limited view, such as of only worldly, tangible matters. Notice that, alone of the Major Arcana, this image has no background, showing that it is isolated and incomplete, without recognition of the greater setting. The figures at the Devil's feet are bound, but not oppressed. In fact, they even lustfully desire the bondage, representing the aspect of domination by desires instead of rational thought, or the acceptance of a bad situation.
Kigai Yuuto is perhaps the most detached character in the entire series, and yet paradoxically he is designated as the Devil. He seems to understand people to some degree, as when he gave Satsuki a reason why it is wrong to kill humans, but with his nonchalant attitude, he dismisses their motivations. He does not believe in driving passion nor spirituality, possesses none himself. Thus he is very limited in his experiences; he will never experience the highest and lowest points of life, merely flow along and let things happen as they will.