After the unexpected lost of her kind in the war, she was left as the last genie in the world. only a few know of her existence, and this few gets even fewer as they battle it out to become her master. -SasukeSakura

She believes that they were called 'exes' to denote that they were the wrong choice. Just like when you get a wrong answer, it is marked by an 'X'. But then again, in maps, 'X' marks the spot.