John Watson.
This is a good name, a solid name. A bit traditional, a bit old-fashioned, perhaps, but this only serves to highlight his unwavering dependability. John Watson—Dr. John Watson—here is a man you know you can trust. He is strong, like his name; he is strong and sturdy and always, always there for you. He is rather like a table, really: firm and steady, he is your support, the one who takes the burdens from your shoulders and puts them on his own. No fuss, no nonsense: you know he will never falter.
He is also a perfect foil to one Sherlock Holmes.
What a superfluous name, Sherlock. Arrogance and vanity seep from its curves and corners; it's a package of pretension and brilliance and a little insanity all tied together with a purple velvet ribbon. It is refined and ridiculous at the same time, perfectly describing the mad, mad genius that is the man who bears it.
Sherlock Holmes is indeed a foil to John Watson, but foils are also complements: to John, Sherlock is vivacity and life and buzzing, simmering energy; he is insane but perfectly flawed, complex and yet very simple. To Sherlock, John is solidity; John is his rock, if Sherlock ever was to use such a clichéd metaphor. John need not be startlingly brilliant, like himself; he need only be there, and there he always is.
Foils, yes; opposites. But they are Holmes and Watson, John and Sherlock, leaning on each other and absolutely, irrevocably a team.