Tom may be Pedantic but I agree with him in general. Whether considered a wild card or not "[a]" is clearly not a character. Despite BOL (see the article for "LIKE") and Steve, the word "Character" is generally understood to be singular and means one character so "[a]" is not a character it is a character string.

As an aside, one of the most common sources of error in our business is misunderstandings attributable to imprecise use of language. This is a trap we all fall in. Whether we are on the specifying side or the interpreting side we need to seek clarity whenever we suspect the possibility of confusion.