As I said, Evan Wilson is an example of the common trope in episodic fiction to do stories centered around guest stars. Serial storytelling revolving around the leads has become so pervasive in the past few decades that we've forgotten how routine guest-star-driven stories used to be, and not just in television. Many of Sherlock Holmes's prose adventures, including two of his four novels, were just frames for extended chronicles of the adventures of different characters altogether, which they narrated to Holmes and Watson in flashback.
A featured guest star is only a Mary Sue if he or she is handled badly or exaggerated to the point of unintentional farce. Evan Wilson doesn't fit that description.