Marv was a wraith and Sal Lassiter's bodyguard.
At some unknown point in time Marv met Sal Lassiter and became his bodyguard.
Sal and Marv went the upscale restaurant Il Secundo to hang out in the private room in the back of the restaurant. When they arrived they interrupted Ennis Ross' conversation with Maurice, the maitre d'. Maurice left Ennis to attend to Sal's needs. Ennis got mad and interrupted the trio. Marv asked Ennis if there was a problem and Ennis left them alone after seeing Marv's refelction in the mirror.
When Sal and Marv arrived in the private club Sal got into a fight with his rival Julian Duval, a werewolf. Marv helped break up the fight and Julian left the club. Moments after Julian left the club it gets attacked by hunter Irv Sokolowski. Marv and Sal were both killed in the attack. Marv was killed when Irv slashes his throat and across his chest with his artificial silver claws.
After being saved from Maurice by the Winchesters, Ennis asked about seeing Marv's true face in the mirror. Sam identified Marv as most likely being a Wraith and explained how a mirror or a camera could be used to identify a monster.