Vile Concoction (referred to as "Connie" by her friends) is one of a handful of witches that live with Mystery Brew in her townhome, as part of a coven who run a potion shop together.

Connie likes to keep to herself, and is often sulking in the dark corners and rafters of the shop. She usually caters to the night folks, and works the graveyard shift.

Her specialty is brewing poisons.
You'd think that in a world of technicolor ponies where most are friendly, there wouldn't be a need for such things. But you'd be wrong. There's still some bad actors roaming around, and whenever somepony needs a poison for their scheme, Vile is the pony to call.

She's a bit of a spiteful prankster and likes to mess with patrons during the day if they disturb her rest, or rub her the wrong way. She often causes mishaps in the shop, whether that be purposely giving a customer the wrong order, sabotaging a mixture to spite somepony, or spilling a bottle onto an unassuming patron.

She's not nice, but she makes up for it with her poison knowledge.
Being somepony who can - at a glance - tell you whether or not your combination of ingredients is dangerous for consumption.
If something smells fishy, she's the first to speak up.

Connie is what someponies would call a "tsundere goth."