The way the Retainer's work is you can hire up to two, you customize their apperance and choose a set personality and name for them, they work as banks and are used to sell items on market boards. You can set each retainer up in different cities, but the items they post are avalible in every city, instead, each city has a Tax rate on the goods it sells, so say Ul'Dah has no taxes set and Gridania has normal taxes set, Sellers will get full price for items sold from a retainer selling in Ul'Dah, but only get 95% of the price of items sold from a retainer in Gridania. You are also allowed to transfer Retainers from one city to another so... it's really a silly system to be perfectly honest.