Alright. Time to write Ze Responze.

Like Mono said, I would like to have her powers fleshed out a bit more. Examples of what she can do, and so on (Yes, you can go full on Pokémon and have her send out a torrent of fire from her beak). Generally, add a bit more description to her powers, and not just "She can do fire." Same goes for the healing. About the healing, by the way, you can make her healing powers a bit stronger. She is a Phoenix after all, and they are one of the very few Mythics who actually have healing powers, so feel free to make them a bit stronger. 

You're lacking a bit in the appearance description as well, and while it's "fine," I'd still like to actually get a bit more details. Speaking of: Purple/Violet eyes is a no-go, I'm afraid. It's just not possible for a human to have that eye colour. So flesh out the appearance description (Ethnicity, and such. Where does it look like she's from, based on appearance? My character, Lily, has black hair and dark eyes, and a slightly round face. Add to that a very slim build and you find that all are traits that are very common in Asia. Lily half japanese). 

Mythic Essence can't be rejected. All humans have magic, but it's like a blob of clay. The Mythic Essence comes in and molds it to a certain shape. Once it is shaped, it can't be changed. Also: All humans are compatible with Mythics, it's just a question of [i]what[/i] Mythic they are compatible with.

Furthermore, the Shifter Program/Twin Soul Program was started ten years ago, so when Cassie was 10. Her getting wounded in the attack when she was 15 is okay, however. This also means that her parents couldn't have been parts of the research team even before she was born. They would've worked on it in ten years, max. It took only 6 months to complete, after all. 

Finally (I think) there is her speed. While birds can fly fast, I think you made her a tiny bit too fast. Reduce the gliding speed by 20 km/h (both minimum and max), make the current one its Flight speed (flapping its wings) and keep the Diving Speed as it is. These are all faster than an actual Bald Eagle, but not as ludicrously fast as before. The Demons are supposed to be the Speed Monsters, not the Fae. 

That should be it, then. Also, +1 bonus point for being the first non-humanoid Shifter :p