At the end of chapter three, Nick claims,

"Every one suspects himself of at least one of the cardinal virtues, and this is mine: I am one of the few honest people that I have ever known" (59). 

However, Nick really seems to think too highly of himself.  He also claims, in the very beginning of the novel, that he is

"[...] inclined to reserve all judgments [....]" (3),

when we, as readers, know this is not the case.  He immediately tells us that he

"[...] [wants] the world to be in uniform and at a sort of moral attention forever"

but that Gatsby is exempt from such criticisms (4), which immediately makes Nick an unreliable narrator, if nothing else.  (Decide for yourself if he is honest.)  He immediately has lied to us, in the first chapter, by claiming to be nonjudgmental, then proceeding to judge others.

I would characterize Nick as untrustworthy -- though if you want another characteristic of him, I would characterize him as weak.  For example, though he supposedly has such strong morals, he does not have the backbone to leave when Tom and Myrtle "insist" he stays for their party in chapter two.  Instead, Tom guilts him into staying with the comment,

"Myrtle'll be hurt if you don't come up to the apartment.  Won't you, Myrtle?" (32). 

In this case, Tom speaks for Myrtle -- but in a way, he also speaks for Nick, because Nick's lack of a response speaks for his willingness to follow and his lack of backbone. 

For more information and a somewhat different perspective, you may also want to see the link below.