Thalia is the average height for her age, standing approximately five feet tall. She appears somewhat Xingese, with long, straight black hair slightly covering her left eye. Her eyes appear black at first glance, but are actually a very dark blue, due to her Half-Amestrian, Half-Xingese origin. Thalia sports a sleeveless black sweatshirt and black pants with black fingerless gloves and boots. On the back of her sweatshirt is a white flamel symbol.
Thalia is often quiet and thoughtful, but tends to make sarcastic remarks. She is portrayed as cold and calculating, as well as vastly intelligent and arrogant. She often looks down at other people, scoffing at their incompetence. Thalia is not often bested, whether it be in an argument or a physical fight, and on the rare occasion that she loses a battle, she's thoroughly shocked. However, when Edward Elric loses against her in a fight, she acknowledges his efforts and treats him with respect.
Thalia's mother was Amestrian, and her father was Xingese. However, she was raised in Amestris for the majority of her life, in the city of Dublith. Her father was killed by Scar when she was ten, after he wandered into an Ishvalen village with the intent to help restore their settlement with his alchemy. Thalia's mother forbade her from using alchemy, telling her that it would only get her killed. Thalia studied in secret for years, and in doing so discovered a new type of powerful alchemy. Already having a naturally high level of intelligence and understanding, she became obsessed with knowledge. Her arrogance grew, until she convinced herself that she was strong enough to bring back the dead. When she was twelve, she attempted human transmutation in the middle of the night, with the intent of surprisng her mother in the morning. When she arrived at the gate, she wasn't overly downtrodden about the rebound, and considered the Gate to be an oppurtunity to learn new information about alchemy. Thalia accurately guessed who Truth was, much to it's amusement, and fully embraced the knowledge given to her through the gate. When she was informed of the toll, her sanity, she smiled and stated, "It's only natural that my most prized possession is taken from me. I respect you, Truth, and I won't disagree with your decision. However, you should know that I'll be back to retrieve it.". When her mother discoverered what Thalia had done, she commit suicide, unable to live with the burden of technically "losing" two family members. As her toll, Thalia has mental breakdowns that involve excruciating pain in her skull. She often calms herself by reciting the elements, but sometimes she loses control and lashes out at people.
Thalia is skilled in hand-to-hand combat, and is a fast and agile runner. She has quick reflexes, and her superior intelligence helps her map out battle plans quickly and efficiently. At the age of ten, she began studying alchemy and came across a realization. Amestrian alchemy used Tectonic Energy, and Xingese alkahestry utilized the earth's natural chi, so what about Dark Matter? Thalia had a theory that since Dark Matter was invisible, and it made up three fourths of the universe, it would be possible to use it's energy to perform alchemy, and seemingly bypass equivalent exchange. She looked into her research some more, and eventually created an alchemy type that she called Tenebris. She was able to form weapons and objects seemingly from thin air, using black electricity. By giving Dark Matter a physical form, she was also able to send off powerful bursts of energy. Ever since the transmutation, she has been able to use Tenebris without a transmutation circle, making her a formidable opponent in battle. However, the more she uses her alchemy, the more often she has her mental breakdowns.