Carth Onasi is a human pilot and Republic naval officer, born on Telos IV in 3994 BBY, decorated in the Mandalorian Wars and known as "Fleet." He is the first companion the player meets in Knights of the Old Republic, written by David Gaider and voiced by Raphael Sbarge.
The mentor who defected
Onasi served under Admiral Saul Karath, who became his mentor and then went over to the Sith and bombarded Telos, killing Onasi's wife Morgana and taking his son Dustil. That is the load his character carries: he is a man who was betrayed by the person he most respected, and who is now travelling with an amnesiac stranger the Jedi Council will not explain.
Suspicion as a character trait
The result is a companion who spends the first half of the game being difficult about it, and who is entirely right to be. Contemporary players found him tiresome; on a replay, knowing what Revan actually is, his instincts read as the sanest thing in the party. It is one of the earliest examples of a BioWare companion whose arc is designed to be re-read.
Afterwards
He appears in the 2004 sequel and becomes an admiral in the Legends timeline, and is referenced in the in-game codex of Star Wars: The Old Republic.