The Opposition

Question: I have my protagonist and his "allies" created but I'm starting to wonder if I've done my antagonist correctly. I understand that an antagonist should be more powerful than the protagonist but should he/she be a sort of foil as well?
And it also sometimes seems that I've spent so much time fleshing out my protagonist that it's difficult to create an antagonist on a similar level.

Answer: Regarding the "antagonist as foil": often in genre or plot-oriented fiction, the antagonist is also the impact character. That is, in addition to trying to prevent the protagonist from achieving his goal, the antagonist also represents a way of being or doing things that is opposite to that of the main character. Thus, looking at the antagonist/impact character forces the main character to question himself.

As Melanie Anne Philips points out, this is why you sometimes hear the antagonist say, "We're very much alike, you and me," to which the main character says, "No, we're nothing alike!"

However, it doesn't have to be this way. Often a story is more interesting if the impact character is someone other than the antagonist. The main character's love interest is a common choice. But the impact character could be anyone - a friend, mentor, hero, rival, etc. who the main character has a relationship with. In such stories, there is less emphasis on the protagonist/antagonist relationship, so the antagonist doesn't need to be as richly drawn.

That said, it's never a bad thing to develop the antagonist as well. It just may take more thought and effort.