Tomás Cantillo was an 11 year-old boy who worked for a local gang of drug dealers in his neighborhood. He was the younger brother of Andrea Cantillo.
Tomás' initiation into the gang involved shooting a rival dealer who was selling on their territory. This rival dealer was Christian "Combo" Ortega, Jesse Pinkman's friend and, at the time, employee.
He was always seen on a bike on the street going around in circles. He was approached by Jesse who buys the meth he carries in his pocket, however he didn't give it to Jesse until money was received by his supervisors. Tomás was later killed after the meeting between Jesse, Gus, and the two drug dealers. The meeting concluded with Gus restricting Tomas' involvement in drug dealing, but it is highly possible that Gus gave the order to kill Tomas.