Thera is a 21-year-old , 5'5 Earth Elemental created by an unknown scientist. Thera's body is made out of stones and different ores like iron, coal, and gold (though there are more.) There's moss running down Thera's right shoulder, up her left thigh, and down her chest. This same moss is what makes up her hair. Here eyes are made of emeralds. She uses leaves to form her clothes, and she can change the color of the leaves at will by the manipulation of the chlorophyll, but she can only turn them into normal colors of leaves (green, yellow, orange, red, and brown.)
Thera is extremely introverted towards most non-elementals. She will avoid confrontation and conversations. If someone tries to start a conversation, she will find any way possible to get out of the conversation. However, when it comes to other elementals, she will become a complete extrovert and purposefully start a conversation with them, trying to make friends. This has not made her many friends, but it has made some friends.
21 years before the events of Undertale, Thera was created through the channeling of magic into the earth. She never figured out who created her, and this caused a lack of adult figures in her life for the first 7 years of her life, and being one of few elementals in the underground, also left her without many friends for about 7 years. Then she found Grillby and figured out that there were more elementals, a new hope lit in her light. A hope that there were more like her and she wasn't alone. Since then, she's considered Grillby as more of a father figure than a friend. And while she's still a major introvert to most, she is friendly and outgoing towards elementals.