who or what kinda person is this Mister In-between in the oldie "Ac-Cen-Tchu-Ate the Positive" by Bing Crosby

Question

You've gotta accent-tcu-ate the positive, eli-my-nate the negative,
Latch on to the affirmative, don't mess with Mister In-between.

Answer 1

It means not to be wishy-washy. To take one side or the other but to make up your mind one way or the other and not stay in between the issues not ever making up your mind.♪