In this made up, fan fictional, story. Robin was dating some random guy, who turned out to be outright toxic. The Ex was just rude, disgusting, toxic, and overall unlikable. 

Nico Robin broke up with him, but then the Ex started being even more toxic and rude. So she hired the You to keep her safe, just in case the Ex did anything. 

Nico Robin promised the You anything they wanted, as long as they helped her. Robin really didn't want to go out anymore because of her Ex.