Scott Stills is an old friend of Skye's from California who came to protect the group, even though he only really knows Skye. He's a very protective-type person and offers a ride every now and then, especially when he thinks there is danger.
He first appears in 11 Part One - Hunt and 11 Part Two - Hunt. He is the driver for the Five in this video.
His next appearance is in 18 - Sam. It seems he is there to protect the Five.
He appears again in Four where he strangles Sam to death and runs after the two of them get into a fight. Filled with immense guilt over Sam's death, he slits his own throat in Five.