Cassian was not a child. He would disavow any such notions at every opportunity. He became enraged at the suggestion, and argued vehemently until his point was made. He was an adult, no matter what he looked like, and he would be treated as such.
But not, much to his intense displeasure, by the Doctor.
At first he thought that Jezebel did it to get on his nerves – calling him 'boy' and so on and patting him on the head. And Cassian retaliated at every opportunity, showing his obvious dislike in every conversation, not hesitating in reporting the Doctor's actions to his superior when they were close to being out of bounds. But as their time together continued, Cassian realized that Jezebel simply… forgot.
It was highly unnerving. Especially since no amount of argument would stop it. He could yell as much as he wanted, and the Doctor would still forget – when he woke in the morning, or after being off doing whatever it was he and the Card master did, or for whole days at a time.
It made Cassian anxious. Surely, Jezebel wasn't that unstable? But all his actions and words pointed to that being the case. Highly intelligent, but… lacking. A little too dreamy. Prone to starring out windows, lost in thought. And letting birds into his room, to land where they willed, the big picture window wide open so the night breeze swirled that long, beautiful hair around him.
He was an angel, fallen from heaven in the worst way.
When Jezebel did consider it, he thought that perhaps he wouldn't forget quite so often if Cassian didn't have so many little idiosyncrasies that tended towards him being a child, rather than an adult. One in particular – encountered most often – was his habit of rubbing his eyes when he'd recently awakened, hands balled into fists, the way small children do when they're far too tired still to stay awake. He thought it was sweet – adorable, even – but he'd keep that thought to himself, in the interest of keeping the peace between them.
Their relationship wasn't strained, in his opinion. Cassian followed directions well enough, although he tended to hum to himself when he had no particular orders – especially when the Doctor was working on something important and requiring his full attention – but he was getting used to the constant loop of Ring 'Round the Rosie echoing softly off the walls of the lab. It was better, he supposed, than the other habit – that of playing with the little throwing knives he had secreted about his person – which made him inexplicably apprehensive. It wasn't as if the boy – older man; he would remember someday– didn't know what he was doing with them, but Jezebel couldn't help but think that eventually Cassian was going to slip up and hurt himself badly.
It was odd to discover that he rather liked Cassian's company. He was remarkably attentive, and asked questions no matter what the Doctor was doing, and payed strict attention to the answers. At times he suspected that this was not exactly because he was interested in what was being said, but he could not think of another reason for the other's inquisitiveness, so he left it alone. He was tolerant, as well – allowing Jezebel to perform some (admittedly unnecessary) surgeries on him because a living person was required, and he always seemed to end up the only one handy.
He worried about that, every once in a while. There seemed very little that Cassian would deny him. It made him curious as to how close they really were, and if he had missed something. But inevitably, something more immediately of interest came up, and he forgot the subject altogether.
Cassian, of course, was fine with that.