Constantly scheming and conspiring against the dumbasses of Ponyville [their words not mine].  
These two aren’t con-artists or school bullies but there are some common ground in their day-to-day lifestyles.  
Together, they have always loved the power that comes with being at the top and rely on each other to get there. They barely tolerate each other but Thistle needs Pixie for his magic and looks while Pixie is far too stupid and weak on his own to be considered a threat.  
They’ve been “friends” since childhood and have been trouble that whole time.
 
Some facts
 
Thistle Trill  
-The daughter of Treehugger and Starlight Glimmer  
-Even without a horn, Thistle has been able to replicate some magic with potions.  
-While she’s not biologically related to Fluttershy and Zecora, due to her mother’s relationship with the pair she does consider them aunties.  
-Zecora saw a lot of evil potential in Thistle from a young age and along with Starlight, Treehugger and Fluttershy’s help has softened that potential to something milder. She’s still a problem child though.  
-Thistle has a little sister name Xanadu whom she hates with a burning passion. She doesn’t get along well with most ponies her age but seems to mildly like Phoenix Fireworks [Twilight’s daughter]  
-She has a pet Possum named Digby who’s her true best friend. He’s a sneaky lil dink.
 
Pixie  
-The son of Trixie and Sunburst  
-Pixie is a hoe. He LOVES the attention of stallions and will flirt with anyone under those parameters [he’s not picky]  
-He’s extremely lazy and never had many goals in life besides getting attention for his looks and following Thistle around.  
-Oddly enough, he does like Sci-fi. It’s the one thing he and Sunburst have in common.  
-He’s a very good cook though as stated before, he’s too lazy to do anything with that.  
-While Thistle is the one who’s kinda mean, Pixie doesn’t seem to understand that he’s supposed to do the same. He follows Thistle around helping her spread chaos but the jury’s still out on if he genuinely cares about what he’s being involved in.