You know, what I really like about Mary's character is that she's actually fleshed out, she's not just one of the numerous nice people who think Kyuu-chan is cute. The fact that she's head over heels for our favorite little cat but still have her mischievous personality is good. And that make her relationship with Aoi pretty interesting. Yes, I ship the two, but i'm still not sure of if she's doing all those craps to him for shit and giggles (and money), because she doesn't like him or because she like to tease people (or especially him).
I tend to think it's the last one, because he really doesn't seems to mind all of that. Like he know he's being played with but he's okay with it, because he know she means no harm. Maybe she's just trying to get a reaction out of him?