The invisibility is mainly temporary and it's more or less like a biological thermal camouflage.  For a brief amount of time he merely *reflects* his surroundings onto him and blends in slightly.  It's only when he attacks, the effect wears off and he becomes completely visible.  I was originally going to have him use darkness manipulation to shroud his body in shadows, but Rebecca already has that ability, so I went with this instead.  It's more effective at night, but during the day, you can see a small silhouette of his figure still.  Nevertheless, he still gives off a heat signature and can be detected by radar and UAV.