Sandbending introvert and ungrateful brat
Shen was born to the single wealthiest merchant in the desert. He was reared by six different maids and various tutors until age eight. At that age, his father discovered he had some Earthbending talent, and sent him to Ba Sing Se to study.
Three years in Ba Sing Se had been the best of his life, living in the upper ring and in general bliss, until he met an old, strong man. His name was Dongyu and had a very abstract way of thinking. For a princely sum he agreed to go back to the desert with Shen as a mentor and Earthbending teacher. In the desert, Shen found his Earthbending useless, and even though his father would pay huge amounts of money to bring blocks of stone to Shen for practice, he would always use them to make more sand.
Dongyu fascinated Shen, and the nomadic man taught him the value of things and the joy in everyday life. Shen asked Dongyu if he understood Sandbending, which he did not. Dongyu, however, told Shen that everything in the world was ultimately made up of the four elements and a few other choice things like love, life, joy, and ambition. Shen was struck by the thought, and started to think very laterally and became a very thoughtful, if not quiet, young man, and a skilled Sandbender.
His father, worried that Dongyu was trying to turn his son against him by not teaching him "real" Earthbending, banished the mentor and had Shen taught in private. By fifteen, Shen's hatred for his father had peaked, and he adapted everything he learned to break out of the stone walls of his estate and set off across the desert. He had been practicing his Earthbending all along.
Since his violent escape from his father's estate, Shen attempted to travel to Ba Sing Se, where he assumed Dongyu would have gone, and has made a point of only barely surviving, having sold all his finer, stolen things, and now forced to rely on bending prowess (however meager) to make a living.