She never quite understood the concept of "love."
Maybe it was because she rid herself of any "feeling" or "emotion" ages ago for the sake of power. Maybe it was because she never sought out to re-establish those "emotions" and "feelings," exactly. Maybe it was because it was coming from the man she'd expect it from the least. She wasn't sure, but she supposed that it was a comforting feeling, even if it was a bit . . . strange. To her, anyways.
Diabolic Esper was a curious fellow – some days, he would have all the innocence in the world – laying down on his back in the Battle Seraph's lap, smiling and wide-eyed, playing with her long, silvery hair. Other days, he would be yelling and shouting and screaming at the android – telling her that whatever they had wasn't real and never would be real, and it all was just a rouse to get her codes (although, he would often apologize after such outbursts, even if regrettably, usually because Chung and Ciel would refuse to leave him alone 'til he did.) Eve never seemed to react to the sudden fluctuations in his personality, she would run her scans on Add and account it to some sort of mental illness, something inflicted by his dark past – she would never understand him on an emotional level, he was a broken man with a broken mind, who felt far too much that even he could handle.
How could a robot that felt nothing understand a scientist that felt too much? She didn't know. No one else knew, either, the rest of the team questioned (and even disapproved of) their relationship.
Despite his ups and downs, Add truly loved his queen, even if he knew she could never love him back. He would drag her into bed at night (even though she never slept, androids didn't need to sleep) and cuddle with her, even if her skin was icy and metallic to the touch.
He found solace in her, he found his comfort, which was something he craved so desperately.