I think- if there was supposed to be a back-story to Mel- She would've been one of the test subjects Wheatley helped. Re-read what he says in the boss battle and went: ORLY?!
"Remember when I first told you how to find that little portal thing you love so much? Thought you'd die on the way, if I'm honest. All the others did."
"You didn't think you were the first, did you? Ahahaha! No. Fifth. No, I lie: Sixth. Perhaps it's best to leave it to your imagination what happened to the other five..."
"You know what? I think we're well past the point of tasteful restraint. So I'll tell you: they all died. Horrifically. Trying to get that portal device that you're gripping in your meaty little fingers there."
I'm a bit attached to Mel at the moment. :1 bite me! Not literally...