The main theme of Daryl Gregory’s “Second Person, Present Tense” is what comprises personal identity. The protagonist, Terry, believes in herself as a separate person from her past self, and the conflict in most of the story stems from her view differing from those of the characters who think of self as continous through a single body.
Somebody else has already written an article on that topic, though, one that’s much better composed and goes into extraordinary amounts of depth and insight that I couldn’t hope to match. So, rather than beating the terminally ill horse, I’ll try to talk about Dr. S’s impact instead.
Dr. S is a major character in the story: he supplies the explanation of Zen’s effects and Terry’s argument for being a different person than Therese, but, just as importantly, Terry is clearly infatuated with him, a fact which she admits to. Terry, on first awakening from Therese’s overdose, is extremely vulnerable emotionally—she has a hand-me-down body and a disjointed set of memories that she doesn’t recognize as hers but nonetheless attaches emotions she doesn’t understand to. Dr. S is the first person she, as Terry, ever has extended contact with, and thus sets the anchor for her own set of memories different from Therese’s. Naturally, she develops an emotional attachment to him. Terry notes at the beginning of the story that she is only two years old despite the age of her body, something repeated by Dr. Mehldau later on, and that she cannot remember clearly her first several weeks after waking up; as far as genuine life experience goes, Terry is still a child. Dr. S, in other words, would have been her first parental figure, and clearly, like most children with their parents, Terry adopts much of her outlook on life from him, from her religion to her view of identity to at least some of her cynicism. Dr. S does essentially exactly the same thing to Terry that Dr. Mehldau attempts, except instead of trying to make her pick up Therese’s life his existence itself causes her to make her own attempt to pick up his life. For all that his philosophy directly opposes Dr. Mehldau’s, and for all that it’s not entirely intentional, Dr. S’s presence colors the entire story.
Also, Gregory characterizes exceptionally well a character who only ever shows up once in the story, using only objects, other characters’ testimonies, and direct memories viewed through a heavily biased narrator. By the end, Therese’s person is just as clear as Terry’s is.