someone who knows everything about some one but denies it and secretly wants them

Creepy Bastard

Stalker.

The word stalker is often now loosely used to refer to anyone who takes a greater romantic interest in you than you do in them. But more strictly, it should be used to an unwanted aspirant lover whose attentions have become obsessive, bordering on or beyond insane. Invariably a stalker will not think of him or herself as such. They'll use one of the following rationalisations:

1) The moral crusader. The victim has done something bad and deserves to be psychologically bullied as punishment.
Literary example: Notes on a Scandal by Zoe Heller (www.wikipedia.org/wiki/Zoë_Heller)

2) The secret accomplice. The victim has given out secret signals that they welcome the level of attention. However much they deny it, they want it.
Literary example: Enduring Love by Ian McEwan (www.ianmcewan.com)

3) The devoted protector. In this very common case, the stalker believes he or she is merely looking out for the victim, keeping an eye on them in their own best interests.

Literary example: Memoirs of a Stalker by Thomas W. Hodgkinson (thomaswhodgkinson.com)

A person who risks his life to loot the ruins of Chernobyl for nothing much in return. Usually wears a gas mask, N.B.C. suit, and is armed with an AK-47. Can be found in groups huddled around a campfire squatting to some tunes.

Someone who follows another person around, becoming obsessed with that person, eg collecting hair, taking photos, observing them with binoculars etc.

when you're looking through you're pictures on a camera, (s)he's the person watching you from behind.

Someone who just won't leave you alone.