The lateral bone in the leg is the fibula. It is a long, thin bone that sits parallel to the tibia. The fibula is responsible for providing stabilization of the ankle and support for the lower leg muscles.
The fibula serves as an attachment point for the strong muscles of the upper leg. This includes hamstrings and the biceps femoris muscle. It is also used as source of tissue to graft for use in repairing damaged bones. There is more mass in the fibula due to the lack of weight it actually bears, making it a key candidate for grafting.