Danny Phantom headcanon that one reason people don’t recognize Fenton as Phantom is because, aside from the fact that it’s not a conclusion most people would easily come to, Phantom doesn’t look very human much less like Fenton.
Phantom is much more solid than most ghosts, easier to see than others but he’s always stuck in a state of semi-invisibility in which humans can see right through him. His whole body looks mostly whole but also kind of fuzzed out on the edges like he’s blending slightly in the environment. He’s most solid closest to his core but as you extend outwards, he kinds of fades a bit. He doesn’t come out well in photos because of this, many ghosts can’t be captured at all.
His snow white hair is much the same, being more like a semi-solid mist instead of hair. It can behave like a solid but it moves as if he’s underwater. The locks that float around him look to be the same consistency of a cloud and only he can really touch it. His trademark glowing green eyes are just that, glowing green. They’re practically like flashlights and are an incredibly distracting feature that casts his whole face in an eerie glow. Most people find themselves drawn to them rather than his facial features. But should you look past the eyes, his face is still hard to describe. It’s like looking at an old grainy photograph without defined faces. You can see his expressions, see him move but key features are hard to pin down. Most people would never describe the two boys as being similar, aside from the names. Even the people in the know needed time to get used to Phantom’s unfamiliar, inhuman features.