Danny sees Leo quite often, and that's the problem. He wasn't supposed to. Leo is the essence of a stone-cold killer, that was injected into Danny's mind somehow (that part isn't very clear).....Leo was only supposed to be 'activated' at certian times to assassinate, etc....But went out of control. After Leo self-activated and killed Danny's wife, Danny (with Leo in his mind) was locked up for 6 years at the asylum, where the story starts.
There are several chapters that take you into the past, to help you understand what Leo did in Danny's body.......Danny doesn't remember, and is trying to find out why. He thinks Leo is a real person, and has forgotten all about the original experment.
When he finally finds out what Leo did.......Killed his wife.......Then the showdown begins.......For the ultimate control of Danny's mind.