The answer is "message" and I will explain why.
This is because reply is not a noun with a specific and direct relative. It is a very general word.
Here are some nouns and their counterparts:
Parent - Children
Question - Answer
Ancestor - Descendant
The difference between children, answer and descendant is that each of them has a direct relation to their precursor. To be a child can only be an offspring of a parent, an answer can only denote the existence of a question and a descendant will have to have ancestors. It's basically law.
Reply has no specific relative because an answer is also a reply but a reply may or may not be an answer!
"Reply" is very general and the only other general word I can think of that is most relevant is "message".
For and against argument:
A parent denotes a child and a child denotes a parent. However, a question does not denote an answer but an answer denotes a question.
Hence, the answer to your question will have to be based on "reply" in order to be 100% accurate with the other definitions of counterparts - the existent of A means also the existence of B but may not be vice versa, yet they are still considered counterparts.
A reply absolutely denotes a message.