Who is the most dynamic character (the one who changes the most over the course of the novel) in the novel?
Darl is probably the most dynamic character in the book, in the sense that he undergoes the greatest change. His defining characteristic is perceptiveness, and that characteristic does remain unchanged. For example, he can understand that Dewey is pregnant simply from observing the way she was crying. Throughout the novel, he also has the most poetic use of language, but his language gradually disintegrates in coherence. The stress of the journey, on top of his war time experiences, eventually drive him insane, but it is an insanity that causes him to leave the family environment that causes him so much distress.
"[Vernon] watches Jewel as he passes, the horse moving with a light, high kneed driving gait, three hundred yards back. We go on, with a motion so soporific, so dreamlike as to be uninferant of progress, as though time and not space were decreasing between us and it" Darl, p. 101.
"She cried hard, maybe because she had to cry so quiet; maybe because she felt the same way about tears she did about deceit, hating herself for doing it, hating him because she had to. And then I knew that I knew. I knew that as plain on that day as I knew about Dewey Dell on that day." Darl, p. 129
"It is as though the space between us were time: an irrevocable quality. It is as though time, no longer running straight before us in a diminishing line, now runs parallel between us like a looping string, the distance being the doubling accretion of the thread an not the interval between." Darl, p. 139
"Life was created in the valleys. It blew up into the hills on the old terrors, the old lusts, the old despairs. That's why you must walk up the hills so you can ride down." Darl, p. 217
"Yes yes yes yes yes yes yes." Darl, p. 244