The weird thing about Dexter is that... people only imagine others who are dead, implying that ghosts may actually exist within the context of the show.
Maybe I'm wrong, I don't recall Dexter imagining someone who was alive...
Oh, but Travis imagining Gellar, would mean that Gellar is likely dead, it is a high profile case, so if there were an innocent Gellar hanging around, someone ought to have noticed.
Or... he could be a ghost!!
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It's common courtesy for a lady to swallow
If you lived here, you'd be pwned by now