What is a "reflection character?" How is s/he different from an antagonist?

My understanding is that a "reflection character" is someone who acts as a "foil" for the hero(ine). This is someone the main character fights with, or at least interacts with, a lot.

How is this person different from an antagonist given that this person fights the hero a lot? Is it because the two peoples' interests are basically aligned?

Put another way, is the reflection character always, sometimes, or never the antagonist?

Answer

The reflection character, as I understand it, is literally someone who reflects the protagonist: someone who echoes parts of the protagonist's character or situation to expose the subtext and make it more visible. The reflector could be an antagonist or an ally, or neither.

On the BBC's Sherlock, S3E1, "The Empty Hearse," Sherlock's parents (cleverly played by actor Benedict Cumberbatch's actual parents, Wanda Ventham and Timothy Carlton) are visiting him. Mrs. Holmes is dressed in a white shirt, dark pants, and a black trenchcoat with an upturned collar; Mr. Holmes is wearing an Army-type jacket and a plaid shirt. They are reflections of Sherlock and John respectively (both in choice of clothing and as the fierce intellectual/laid-back anchor). Sherlock may bicker with or dismiss his parents, but they aren't antagonists — they aren't trying to prevent him from doing anything.

So "the reflector" and "the antagonist" may or may not be the same character; they are two different roles, which one or more characters can hold. You can have many different reflectors and multiple antagonists.