Once again obsessing over Vigilcourt, the thinking man's ship. I think they'd see eachother as the only people in the group that are truly their equals.
For Adrian's part, Harcourt is the only one who doesn't baby him. And while I firmly believe he enjoys being handled with kid's gloves because it lets him get away with doing whatever he wants, he'd definitely notice that Harcourt doesn't do that and be drawn to her for it.
And for Harcourt, Adrian is the only person she wouldn't see as having the high ground over her, whether that be morally or emotionally. In a fairly self-loathing way, I think she'd see alot of herself in Adrian. She doesn't have to walk on eggshells around him in fear that he’ll see her true self and be horrified, there is no dark corner of her heart she could turn over that he'd be shocked by because he's worse.
They are the only ones they can trust not to hold back on eachother when they spar, the only ones they know won't flinch away from eachothers violence. I think they'd both take alot of comfort in that.