Estelle

 Demons and vampires (along with other monstrous creatures) were spawned from the lowest pits of the Underworld, with no access to the moral world, as the Gods sealed them off to prevent chaos and havoc that they will cause upon Earth. Ever since, they have been trying to figure out a way to break that barrier and invade the mortal universe ever since the mortals have defeated the titans along with the Gods and banished them to Tartarus, any 'evil' born deity or creature was automatically sent to there, where they lived a miserable life and were punished severely for their sins, with no chance to live and happy afterlife. Which means that even living immortals are banished to the Underworld, along with other evil souls.

 Estelle was born to a demon father and vampire mother, making her a half-blood in the Underworld realm. There were many half-bloods in the Underworld, yet they seem to not have any special powers, just half of the powers from each of their parents, not full powers from each. So how was it possible that Estelle inherited full vampire and demon powers, along with many mysterious powers that were never seen before? She also didn't look like any of the half-demons & half-vampires in the Underworld, but instead like a human, with the exception of her deep purple irises. Estelle took it upon herself to find a way to break into the mortal world. While it almost incinerated her, she managed to channel her being across Elysium, Asphodel Meadows, the gateways of the Underworld, and magical barrier into the mortal realm. Upon doing so, the Gods noticed that their barrier was destroyed, and that someone from the Underworld broke free. So they quickly sealed Tartarus off from any other part of the Underworld, with Hades keeping guard, and resealed the barrier. They casted a hex where any other creature from Tartarus that traveled the same method that Estelle did will be completely demolished from the face of the universe. The only way anyone else can escape is if they traveled to the mortal world right from Tartarus, except, there was no such method. Until one day, Estelle managed to open a portal that brought her across California all the way to Greece, before disappearing a couple seconds afterwards. Estelle came up with a plan so risky you would have to be courageous, powerful, and stupid enough to attempt. She was going to try to open a portal linking the ancient grounds of Greece to Tartarus long enough for other creatures you escape into the moral world. It seemed impossible, but Estelle, a half demon & half-vampire, was already standing on mortal ground, so nothing seemed impossible at that point. With the Gods searching for the runaway creature, and the amount of power required for her plan, Estelle will have a lot of challenges in front of her.