Okay, so, here's the thing. Russ is a hilarious short character, and the fact that he's played by David Schwimmer (albeit not credited) is utterly side-splitting. There was a lot of fun to be had with the character, and the show certainly took advantage of that. But, he's nothing more than a joke and shouldn't be taken seriously as one of Rachel's boyfriends.
Russ came along in the tenth episode of season two and shocked everyone with his clear resemblance to Ross, which neither Rachel nor Ross could see. This led to some awkward bickering and confusion before Rachel finally saw it and ended the relationship.
In the end, he simply proved that Rachel still had feelings for Ross, and the gag of him looking precisely like one-half of the Gellar siblings made him a very one-note character. Still, at least he found love in the end after running into Ross' ex Julie. A happy ending all around!