The character of Eustace Scrubb, while on the one hand a commentary on "progressive" parenting and schooling--he is self-centered, cruel, spoiled, and, and is at one point described as heading towards an interest in vivisection, which Lewis deplored--Eustace is also the author at a young age.
To begin with, his name. "Once there was a boy named Eustace Clarence Scrubb, and he almost deserved it." Lewis hated his own name--Clive Staples Lewis--and prefered to be called "Jack." He gave Eustace a horrible name to parallel his own.
Secondly, there is the subject of Eustace's general religious nature--namely, that he has none. Lewis was an atheist until he was an adult; it was ony after World War I that he even began to question this stance. Eustace represents Lewis' youthful, atheistic, abandoned self.1.
1. It is interesting to note that the figure who encouraged his atheism--a college professor, IIRC, and mentor--was later to become the model for Digory Kirk