Kyle Katarn is a fictional character in the Star Wars expanded universe, who appears in the five video games of the Jedi Knight series, the video game Star Wars: Lethal Alliance, and in several books and other material. In the Jedi Knight series, Katarn debuts in Star Wars: Dark Forces, appears in Star Wars Jedi Knight: Dark Forces II, is one of two playable characters in Star Wars Jedi Knight: Mysteries of the Sith, appears in Star Wars Jedi Knight II: Jedi Outcast and is a major NPC in Star Wars Jedi Knight: Jedi Academy.
Katarn was originally a member of the Galactic Empire, before becoming a mercenary for hire. He regularly worked for the Rebel Alliance and later became a member of the New Republic as well as a skilled Jedi and an instructor at the Jedi Academy, second only to Luke Skywalker.
Source: Wikipedia, "Kyle Katarn", available under the CC-BY-SA License.