The character known as Thirteen is named Remy Hadley, so her first name is Remy.
Thirteen joins House's diagnostic team after a competitive selection process and becomes one of its most memorable members. The nickname comes from her numbered status among the applicants competing for limited positions, rather than from her given name. Olivia Wilde played the character.
Remy is a physician with a specialty in internal medicine and a personal history that becomes important to the story. She carries the gene for Huntington's disease, a hereditary neurodegenerative disorder, and later learns that she has inherited the disease. Her uncertainty about the future influences several major character decisions.
Because the show usually calls her Thirteen, viewers may overlook her legal name. Her surname is Hadley, and “Remy” is the short form used in the series. She is also sometimes confused with Amber Volakis, another physician associated with House's team, but they are separate characters played by different actors.