reason:
They're gal pals. Rouge is a jewel thief/secret agent/treasure hunter and topaz is a government agent? Cop? Exact occupation a little unclear. But they get assigned to work together and don't get along at first, insulting each other but soon learn to respect each other and quickly become friends, often seen hanging out together off work.
They even go on a cafe date together. Rouge outright flirts with topaz at times. Topaz even gives rouge a ring and blushes about it. Also that scene of them in the vent together. And when rouge has to leave earth and topaz thinks she won't ever see her again, she screams and sobs. Also just for clarification, rouge is canonically 18 and also owns a bar, so she is an adult.