Although Hartigan is quite the obnoxious one, Lady finds it a bit different from how everyone else does the thing.
Sure, he would try to embarrass her and stuff. But his contempt is not really directed to her. Whatever his deal is has nothing to do with her being a commoner in the past or how she is not fit to be a part of the family. He is not degrading her, in other words.
But Hartigan does have a huge dirt on Fallon.
"Oh, what woe! What're you going to do about this now? Your dearest little cousin is falling for me. I might end up taking her away from you if she ever starts to come after me!" he cackled.
"As if she'd come after some imp who'd shamelessly claimed she's still using diapers," he scoffed.
"I appreciate you defending me, Fallon, but will you please drop the whole diaper thingy already?" she deadpanned.
The three of them are beginning to make a scene at this point.
