He was a con man.

He went from place to place, each time with a new name, a new identity. He spoke differently, had different interest, had different hair and clothes. No one could hunt him down.
He took advantage of the fact that he looked younger than his age. He wasn't sure what age he was, but he was sure he was older than the fifteen years he usually posed as. Innocent girls (or boys; it didn't matter to him) with rich parents would fall for him, with the personality he had tailored specifically to their interests, and they'd grow close. They'd share everything with him. Conversations. Movies. A bed, sometimes.
Money.
Then he'd run off, breaking the kid's heart and taking the money with him. He did this from town to town, stealing from those who trusted him.
He couldn't remember why he did it. He just wasted it all anyways.
He couldn't remember a lot of things, to be honest. He couldn't remember who his parents were, or how he'd been separated from them. Had he ran away? Did they die? If so, how? He couldn't remember his name, either. He'd long ago learned not to get attached to names. A name used twice could be heard twice, and tracked down from that. He'd end up in jail. Maybe worse.

So when he came to Amity, he knew the Mansons were rich. They might have kept it secret from the locals, but few things were secret from him. He knew about their daughter, too. Tree-hugging, eco-loving subculture freak.
He could pull that off.
He researched what he could about the goths and about eco-freaks. What the trends were, what they talked about, what they did. After a bit of that, he decided maybe he could pull off a white goth.
It helped that (when he didn't dye it) his hair was white anyways.
He got close to their daughter, Sam. He wormed into her heart, getting close enough for her to trust him completely, totally, with absolutely anything. He just hadn't counted on him.
Either of them, really.
If it hadn't been bad enough that the geek, who just so annoyingly knew who and what he was and liked and reveled in it like a dog in something rotting, followed him around constantly, there was Sam's former boyfriend, who just seemed to X-ray-vision through his gut and knew he wasn't for real, constantly eyeing him suspiciously.
And then, along with all those government creeps in white suits running around, was that kid. The one the suits were after. The one the suits thought was him.
Black and white clothes. Neon green eyes. White hair.
Rather freakish that he was in the same attire when he entered Amity Park.
Danny Phantom, wasn't that his name? How, when the vigilante had looked him in the eyes, he felt in awe at the fact. The fact that this wandering spirit (if that was to be believed), despite all the names that others threw around at him, could still maintain his own constant identity. Danny, his personal name, the one that would be used by anyone who knew him personally, if anyone did. Phantom, his family name, the name that linked him with others of his kind.
Eventually, he blew his cover. No worry, just hide under another alias: Elliot from Michigan, a simple ladies' man. Nothing, more, nothing less.
Yet he still could feel the ghost's eyes boring into his.