The theme in Crispin: The Cross of Lead by Avi, is that of family and what it means to be family. This is aptly demonstrated by the characters of Bear and Lord Furnival. Bear, who is not related by blood, becomes more of a father to Crispin than the late Lord Furnival who upon learning of Crispin's mother's pregnancy kidnapped her and hid her away in a tower until after the birth. He made his own blood relative suffer because of his embarrassment and shame. In the end, the message is clear: blood is not thicker than water. Love is.
