Professor Membrane. Who would latch onto Dexter as the son he never had, which would make Dib jealous and Dexter happy that other scientist of such magnitude is impressed by him. The Professor would keep interfering with the plans to capture Zim and taking Dexter to his lab, to cool science places, to that one theme park he heard was really cool (because Dib had always wanted to go there)
Eventually Dib blows up and has a fight with Dexter and they both try to capture Zim but keep messing each others plan. Eventually they patch their partnership and come up with a foolproof plan to capture the alien... Only for Dexters dad to come up and tell them that it was great spending their family vacation in this city but they have to back.