Dark
When he found out that she sleeps with the lights on whenever he's gone and started to laugh, it wasn't to be mean. It just hit him as funny that a girl who's learned to fire a gun without a flinch at the noise or the blood, who's spent more time spying and seducing and bribing and hacking into confidential systems and files over the past year than she has sleeping, can possibly be afraid of the dark.
When she glared icily at him, wondering aloud if she should tell him that it's for fear of the ghosts of those she's helped to eliminate because it's all part of the job, he stopped laughing immediately.
Since then, he makes it a point to take her along when he needs to be away.