Ok, there's the Dark side definition of Revan, and this is the Light side version. Revan's true actions, personality, history, and even identity were lost to time and myth. But it is absolutely certain that this is true:
Master Revan was the wisest Jedi/Sith to ever grace the galaxy. He was a Jedi, but abandoned the Order to join the Mandalorian wars in 4006 BBY (BEFORE BATTLE OF YAVIN). He then "fell" to the Dark side (according to a conversation between the Jedi Exile and the fallen Jedi Kreia, Revan cleverly orchestrated the Republic's "demise" to draw out the true Sith and prepare the Republic against a potential war with the Old Sith Empire) and harnessed the power of the ancient space station built by the Rakatan Empire in 20,006 BBY: The Star Forge. His apprentice, Darth Malak, was also a fallen Jedi who followed Revan into the war. Malak betrayed Revan from afar when his masters ship was boarded by a Jedi strike team in the Bpffash system. After having his memory essentially rewritten by the Jedi Council, Revan then returned as an avatar of the Force and exacted his revenge on his former apprentice. Revan then left republic space, leaving the Republic to fend for itself. After Lord Malak's defeat, the Sith Empire collapsed into civil war in the absence of a Dark Lord, and Revan disappeared in the Outer Rim. No one know where Revan roams now, not Bastila, not Carth, not even T3-M4. BUT REVAN LIVES!
Revan was the true Sith'ari, the legendary messiah of the Sith, said to walk the path between the Light and Dark sides of the Force, while being neither Sith, nor Jedi. It was he who brought balance to the living Force, but not without the help of millions of Republic and Sith soldiers, and especially the companions he traveled with.