This is reminiscent of my original design for my characters, where Sol, Gavriel, Naidir, and Shira were all Rat demons from the underworld. They had long black tails with sharp ends.
This idea I have now is that everyone is represented by an animal. Their tails appear when they have very intense emotions (Sadness, joy, anger, fear, stress, etc.) and their tails are very sensitive to pain or touch.
Shira is a cat, Gavriel is a rabbit, Grace is a mouse, Sol is a snake, Naidir is still a rat, and Karmine is undecided.
Yeva, Keith, and Maka don't have the "Demon Gene", though, it's not really demonic, more like enchanted.