Dio: E-eh?!
Mary: …?
Dio: M-Mary, w-what are you doing-
Mary: [cocks brow] Mn?
Dio: M-Me? Oh, uh… I was just uh-
Mary: Mn.
Dio: I know I haven't replied to moms letters but I'll be back home soon, I swear. Just a few more classes and…
Mary: Mn.
Dio: You won't tell?! Really?! You are the best, sis!
Mary: Mn.
Dio: Don't you worry about it! I won't tell papa about your… whatever you're doing as long as you don't tell them about my… things. Deal?
Mary: [Nods] Mn!
I've never drawn Diopside before in a scene so here is one.
Dio and Mary are Fluttershy's kids and are the youngest of the bunch. They share a father (Sombra) but are generally distant from one another. What they do share, are secrets.
Mary, due to her fathers dark magic coursing through her veins, has a talent for finding lost and ancient treasure that seems to be drawn to her. Where ever she goes, adventure follows, forcing her to trek through perilous dangers and face monsters hidden in the dark. Mary has no problem with this and treats it like a career to enjoy.
Dio, on the other hoof, is a spy in training under the command of Empress Luna and her team. A lot of stuff is on the down low but his talent in dark magic makes him ideal for dealing with items of interest too tainted to be handled by normal unicorns. In this instance, he and his sister happen to be after the same item and must now work together to get it somewhere safe. It's… awkward to say the least.
Note: Mary is a mare of few, if any, words. Most of her responses are 'mn's and nods. Despite this, those close to her have little trouble understanding her meaning, so she never bothered to try and do otherwise. As such, when speaking with others, she tends to just stare at them with a near unblinking gaze that scares most ponies. Dio doesn't notice and thinks her 'stare' is cool.