Tessa's father is an Eidolon Greater Demon, specifically a fallen angel. Sent by Axel Mortmain, it took the form of Richard Gray and slept with his wife, Elizabeth. Elizabeth then gave birth to a new kind of being: Tessa, a half-demon, half-Shadowhunter, whose birth was made possible by the fact that Elizabeth was unmarked in combination with the fact that the angel Ithuriel, who was trapped in her clockwork pendant, protected Tessa from death. Tessa only ever heard about him from Mortmain in 1878, when she was sixteen. Until then, Tessa grew up thinking her father was Richard.