Willow

Willow is a Will-o-the-Wisp and the first and most powerful familiar of Nero's. Will-o-the-Wisps are magical beings that are by nature essentially immortal, though normally they end their life cycles by becoming one with the place they called home and fertilizing the land; but Willow became Nero's familiar and instead lives within his heart in a dreaming state until it is summoned.

Species
Will-o-the-Wisps as a species are odd. They do not communicate the way other intelligent species do, but they are in fact quite intelligent. The way they speak is through emotions - which made Willow the perfect match for Nero when he first found it. Since Will-o-the-Wisps are essentially living balls of fire Willow has no true gender but is generally regarded as female, hence the name.

It is unknown how old Willow was when she was found, and as a living ball of fire her shape and size can vary greatly despite having no actual mass. Will-o-the-Wisps are born of magic - magic that has gathered in an area (usually a forest) gathers together and eventually gives birth to these nature spirits. They will generally linger around the area they were born for an unspecified amount of time before joining with the land and fertilizing it, ending their "life cycle".

About Willow
Willow herself is something of an oddity among the group - since Nero is the only one who can understand her. She can try to get the attention of others by moving erradicly and leading them places, but that's about it.

Oddly, even though Willow is regarded as female she represents "The Magician" arcana because she is the first form of Nero's magic, having been his first contact with true magic.