Shade has been Ignifex's servant, and in fact one of the halves of the last prince - Marcus Valerius Lux - with all the malice and hatred removed. Eventually he reunited with Ignifex and regains his body.
The Shade aspect of Lux was described as a ghostly pallid man with a face identical to Ignifex's, but much gentler and kinder. His eyes were bright blue, as in his human form.
Firstly Shade seemed to be genuinely gentle and helpful, especially towards Nyx. However, nine hundred years of service for Ignifex had drove him somehow mad and desperate, causing him to kill five of his master's wives trying to show them the true events in the Heart of Fire.
Shade had been created when the Kindly Ones had prince Lux split into two beings; Shade was one of them, the creature with all the malice removed and unable to speak about the two halves of a whole. Since then, for nine hundred years, he had been a captive and servant of Ignifex, taking human form only during the night. Trying to undo the Sundering, he had five wives of Ignifex killed in the Heart of Fire, as they could not bear the truth.
When Nyx arrived, the two of them had at first formed a friendly relationship. Nyx had been thinking Shade was her ally, since both remained captives of Ignifex. Shade had shown her the Heart of Water, where they had shared their first kiss. It didn't take Nyx long before she found out Shade was the last prince, who had survived the Sundering.