Despite their many physical differences, it is clear that Hyde is a manifestation of a hidden element of Jekyll. In this sense, they are foils for one another. This conclusion can be drawn from the initial description of Jekyll in the book:
[He was] a large, well-made, smooth-faced man of fifty, with something of a slyish cast perhaps, but every mark of capacity and kindness.
The description of Jekyll is overwhelmingly positive on balance, but there are a few worrying details. The reader is immediately shown a little sliver of something unattractive in Jekyll (“a slyish cast”), which is hidden, obscured, or overshadowed by his more pleasant qualities. Of note, too, is that his features are described as “smooth.” The same phrase is used to describe Hyde’s housekeeper, an old woman who has “an evil face, smoothed by hypocrisy.” There is more to Jekyll than his clean cut appearance suggests, and the parallel between these two descriptions suggests that this obfuscation is well calculated.
Hyde is a true foil to Jekyll in that he illuminates aspects of Jekyll’s personality that would not surface otherwise. Stevenson’s desire to explore this side of Jekyll parallels Jekyll’s own desire to create and free his double. Hyde’s status as foil to Jekyll also opens up a powerful exploration of the core theme of duality in the text. It is clear, from this excerpt, that Hyde is a direct product of Jekyll’s interiority, rather than his perfect opposite. This idea challenges the easy binary between the “good” Dr. Jekyll and the “evil” Mr. Hyde presented throughout the novella.