[analysis]
'Kay, so, I'm sure this has been said many a times before but I just wanna throw my two cents in.
I know everyone always jokes about how Fenton and Phantom being the same person is so obvious, it's insane no character in the show ever realizes without there being a whole-ass reveal. And I may be reading too much into a children's cartoon's side characters, they could just be oblivious because that's what writers can get away with in a children's cartoon - but if you look at Fenton from the characters' perspective it's honestly understandable no one ever found out during the show's runtime.
Sure, to us it's obvious as fuck, Fenton - Phantom, same voice, basically the same appearance but with a color swap, same dumb puns and whatnot. However, to the cast, Fenton's just a loser kid with his small friend group. An easy target for bullying and a kid with passable grades at best. Compare that to Phantom, someone many of the student body idolize. He's an insanely strong, inhuman, probably dead teen ghost. That, and Phantom appeared around the same time people first started acknowledging ghosts as a real phenomenon. There's no way any human in their right mind would connect this newly discovered phenomenon to a seemingly human kid. It's absurd! The idea had always been too outlandish to even be considered by most. How could they theorize that a human already got mixed with ghostliness when they'd just been told what ghosts are?
Of course, as Danny became more ghostly even in his acts as Fenton, that's when some of the quick-witted students might start to catch on that he's even odder than he was before. They might even start to catch on to how other ghosts' attacks affect him differently, or not at all. But most would still not dare approach the idea that he is Phantom himself. Perhaps a select few will, however it does makes sense that it takes a reveal for most people to genuinly believe/accept Fenton and Phantom to be the same kid. Maybe a lot of people have indeed had a thought like this in the back of their minds but have always kept it there. Not daring to believe this outlandish idea.
He's hiding in plain sight, and it makes sense that he does. He knows no one will even try to link Fenton and Phantom unless he screws up and displays his ghostly prowess in front of someone. That the two are one and the same person, is only obvious to us as an audience because we see both Fenton and Phantom's perspectives. We see him transform and we see his adventures as both human and ghost. That, and it's entertainment media. We would know even if they didn't plainly show them being the same person. That's different for the characters who have known Fenton for the majority of their lives, however.
