In the episode "Doubt" (S01E11) of Wilfred (the US version) Ryan meets a guy who introduces himself as Bruce and who can see Wilfred the way Ryan does. He tells him that he's been Wilfred's friend once, similar to Ryan, but that Wilfred ultimately ruined his life. While he first believes Bruce (after Wilfred once again played one of his tricks on Ryan by luring him into a situation with a sabotaged condom, like he did with Bruce before), during the final conclusion when Wilfred and Bruce fight with each other he saves Wilfred by shooting Bruce. But then it turns out that this was all staged (and the gun isn't loaded with real bullets) and seems to have been a test for Ryan's trust in Wilfred, which he finally passed.
Now given the premise that Wilfred is in the end something like a mentor (a pretty weird one, though) for Ryan to get control over his own life again, and given the title of the episode, a pretty near interpretation would be that Bruce symbolizes Ryan's doubt in Wilfred's ultimate goodwill and his overall purpose (and in the end Ryan's beliefs about Wilfred's intentions are permanently put to test). But while Wilfred is in the end a real dog others can see and whose behaviour is observed by others, too (albeit not in the same way like Ryan does), I'm not sure what Bruce then was. If he was a completely imaginative entity purely created by Ryan's own doubts, then how could Wilfred, who is in fact still rooted in the real world (even if the particular expression of his behaviour was an imagination of Ryan) interact with him? But if Bruce has an actual real world counterpart (like Wilfred's actual dog nature), then who was this guy?
Maybe this whole question is intended to be unanswerable or I'm just making more out of it than there is to it, but it would be really helpful if anyone could shed some light on what Bruce actually is and maybe where the border between Ryan's imagination and the real world is in general. (Note that I haven't seen the last episode of season 1 and all the following seasons yet, but don't mind spoilers either if they can shed some light on this.)