Martin was one of the harvest workers in District 11. He worked alongside Rue in the orchards. He was mentioned by Rue as having a mental disorder when Rue and Katniss were talking about their districts and the see-in-the-dark-glasses Katniss received from her backpack. Rue tells her that they are sometimes given these back home so they could work in the orchards after nightfall. Katniss thinks back to Greasy Sae's granddaughter who has a similar disorder to Martin. She mentioned that she was treated as sort of a pet, and that people at the Hob were always chucking her scraps of food and other things. It was mentioned by Rue that he was a child, but he acted like he was three. When Katniss was told about Martin's death, she comes to the realization that District 12 is sort of a "safe haven" compared to District 11.
Death
He tried to keep his see-in-the-dark glasses by hiding them in his pants. As a result, he was executed right on the spot by the Peacekeepers. According to Rue, the boy probably just wanted to keep the glasses to play with, as he wasn't considered a dangerous person.