If the canon character is acting like he's been possessed by aliens but the plot doesn't seem to explain that, then either something's wrong with the writer, or the character is One Of Us. Sometimes what appears to be a gross inability to grasp the nuances of characterization is merely the sudden and unannounced intrusion of a member of the Special Mary Sue Covert Task Force.
There's this really cool OC that pops up in a lot of HP fics. He's suave, snarky, handsome, a bit of a git, but super good-looking. For some reason, people keep calling him Draco Malfoy.