Agent. Negotiator. A middleman who goes between two sides in a dispute to mediate an agreement. A go-between could be someone who abets a man to romance a woman, a go-between could be a spy who gives information to the enemy. Any situation where someone works between two people or two opposing sides. (Compare to Dicker or Deal maker.)

“They were so angry at each other during their divorce and they wouldn’t even speak to one another so I had to be the go-between to help settle details.”