I agree with lottoman, that it's not a good sign when someone does such things (ie: how can you be sure they won't do it to you -- like the classic "see how they treat a waiter or other service-worker" test) and also that it's hard to tell how bad it is because many people do it to varying degree. Perhaps said friend views your friendship as "real" and the others are just acquaintances (not that it makes it much better, but maybe it makes your personal information more secure - ie: they're not ok telling things about you). Without knowing the exact details of what sort of things are said it's hard to know how malicious everything is (ie: what exactly is meant by "complain about" - as I've joke-complained to casual-friends about how my best friend has a habit of cancelling on get-togethers at the last second (and they laughed & agreed he does have a habit of that) --> it's technically a complaint but it was more of a venting-statement than maliciously saying something behind his back).
If it makes ya feel better, I was recently paranoid that my best friend made up something negative said about me by a girl who rejected me (she never did it to my face, I found out from him - so I had to trust the info he had) -- my reasoning that he didn't want to keep dealing with all the stuff I was "pouring" on him (post rejection) and thought it would get me mad @ her & move on (as his advice had basically just been "put it in the past" "move on", etc etc). Though to be fair part of that thought could also be because bad feelings (rejection) came via other people & my memories w/her were happy-fun times so I couldn't accept she would feel that way/say-something negative like that.
(the feelings died down, but there's still a kernel left - if only because he's still being distant & we haven't talked much at all, let alone about my 'troubles' --> so I can't suss out if he's still receptive or not (and if we do start talking frequently again there's not a normal way to bring everything back up "hey, let's go back to all the that stuff that happened to me weeks ago).