Arma is a white Shinigami with feathers. She has a bright-blue left eye, a small nose, and dark lips covering small, sharp teeth. Her upper torso appears to have an insect-like exoskeleton, and the casing around her body resembles a chrysalis. Her arms are long, extending from the bottom of her torso almost to the ground, and each hand has four fingers.
Like other Shinigami, Arma really enjoys a particular food, this food being grapes. Arma carries an unusual interest in Ryuzaki, going to such lengths as writing an individual's name in her Death Note to protect someone close to him, killing herself in the process.
In the film Death Note: Light Up the NEW World, Arma is first seen when Ryuzaki returns home. She welcomes him back, and they briefly talk.
When Ryuzaki and Mishima escape gunfire, after it is revealed that Mishima is really Neo Kira, Sho Nanase confronts Mishima at gunpoint for killing her brother. As she is about to kill him, Arma writes Nanase's name in her Death Note. Ryuzaki screams as she dies, turning into dust, and she says that she wanted to save someone important to him.