Jimbo is right - that's the 'Mark of the Twelve', otherwise known as the 'Legacy Mark' - it's only given to characters who were originally created in the original version of FFXIV (version 1.0) and who where then transferred into ARR - new characters that start in ARR from scratch do not have this mark.

It's just a way to show that character was from the original FFXIV, and can be turned on and off at will. Hence, it's not something you can get now I'm afraid.