Agatha is a seemingly human woman standing at 5'7"... until you see that her eyes are a solid bright yellow and that there are black horns sticking through the pointed hat she wears. Sometimes she decorates them to look like antlers. She does have a tail, but it's hidden under the long skirt she wears. She prefers darker clothing, mainly blacks and reds - this matches her hair, which is either black or a very dark red depending on the lighting. And around her neck is a circular amulet bearing the symbol of a seven-pointed star.

Agatha has a haughty, uppity demeanor, with a tendency to bravely walk into most any situation... unless it involves water, because she can't swim. She seeks balance in all things, and to bring things that hide in the dark into view. Despite having a tendency to hide her own infernal characteristics, for some reason. But this can also mean that she has a difficult time taking sides on things, and will adapt to whatever company she's with as far as their ideals go... so long as she doesn't compromise her own faith, which is all-accepting anyway.

Agatha, and by extension the Marshall family, hails from a town that flanks a river which has black waters and is rumored to be a very long shadow crossing. Tieflings were held under suspicion in this town, thanks to all the superstition involving fiends and undead alike. The family (otherwise human) was associated with the local temple of Kelemvor, but formed more of a military force dedicated to staving off undead and sometimes fiendish threats from the river - so they were primarily paladins. So when a tiefling was born into the family, they did their best to keep her hidden from the town to preserve their reputation.

As such, Agatha wasn't allowed to be a paladin, and in her early days tended to the weapons and armor they used. But then, over time, she met friends. Her first friend was a fledgling wizard named Serena, and would soon include a satyr named Carnation as well. All of them were outcasts in some respect, and didn't like the religious oppression in the town... so they formed their own religion, one that saw all gods as important in some aspect. The seven-pointed star that functions as their symbol represents the primary forces that encompass the world, represented by the inner planes - the elemental planes (air, water, fire earth), the positive and negative planes, as well as the Ethereal Plane. Together they are the Sevenstar Coven, hiding under the guise of a trio of hags to avoid people looking too much into them.