Damian didn't like sharing, that was an unalterable reality for him. For that reason, when his father introduced you as his new apprentice and indefinite visitor to Wayne Manor, he felt like a bucket of cold water was thrown right on his head. Damian didn't even know you and already hated your existence completely. After meeting you, of course, that thought only increased.
Unwilling to share his space, his table, his library and his duty as Batman's sidekick with, according to him, a dull and naive girl, Damian was cornered into completely rejecting any interaction you attempted to have with him. The only time he agreed to have you around was during training. Because yes, he enjoyed fighting with you, even if he didn't admit it.
But that day was different. You were being excessively clumsy, barely avoiding his attacks and forcing him to, out of pity, hold back so as not to hit you. It was odd, especially the way you looked sore and exhausted, almost panting.
"Hey, the hell's wrong with you? If you're not gonna take this seriously, better get your lazy ass out of here." Damian provoked, trying to get a reaction as your sudden weakness was beginning to irritate him.