What is Watson's relationship like with the other characters in the novel?

Dr. Watson is the narrator of The Sign of Four, and we are left in no doubt through his narration that he is a loyal, faithful companion to Holmes. Watson appears at times to be almost in awe of the master detective, regularly expressing his astonishment at Holmes's legendary powers of deduction:

I assure you, Holmes, that I marvel at the means by which you obtain your results in this case.

Watson is very much an emotional man. This provides a nice contrast to the coldly rational and unfailingly logical Holmes. Watson's rich emotional life finds its clearest expression in his relationship with the delightful Miss Mary Morstan. Watson falls for her and in a big way. Yet Watson is also a deeply honorable man, and so he is keen to avoid the slightest suspicion that he might be after Mary's money. Watson's sense of propriety is also much in evidence in his anger at Thaddeus Sholto for his offhand announcement of Captain Morstan's death.
But in relation to Miss Morstan, Watson's romantic streak can never be held in check for long, and it comes as little surprise when he asks for—and receives—Mary's hand in marriage.