Thalia Halpert-Rodis is one of the students in Bell Valley Middle School and one of the friends of main character Tommy Frazer in the 59th Goosebumps book, The Haunted School. She is also one of the members of the missing class of 1947 who was trapped in the colorless dimension, Grayworld, for 50 years.
Not much about her is known including who her parents were or where she lived in Bell Valley prior to being trapped in Grayworld nor anything else. The only thing is that, like the other twenty-four members of the missing class of 1947, Thalia was one of the kids sent to the colorless classroom by the mysterious and evil photographer Mr. Chameleon for unknown reasons. She managed to retain her sanity while most of the other kids went insane and started a weird and terrifying cult outside the school due to being without color for so long.
One day she discovered her lipstick buried in her purse for so long still had its red color. Excited by this, she started drawing on the walls with it, and to her shock it made a hole in the wall wherever she spread it. Without thinking, she created a red window, climbed through it, and escaped. However, the hole vanished before she could return for her friends. Despite being back in the modern world, Thalia discovered she was still without color and totally stuck in her gray form. She began covering herself with thick make-up and lipstick. She also began using blue contact lenses to conceal her gray eyes and dyed her hair blonde. This was a constant state and it was because of this, that Thalia was endlessly teased by the other kids of 1997. Despite being forced to hide her otherworldly condition, Thalia still helped Tommy and Ben on the local Dance Decorations Committee, but never forgot about saving her friends so she mostly stayed in the school searching for a way back into Grayworld.
Eventually once she does return to Grayworld (by way of the hidden boarded up elevator in the old school building). Thalia eventually decides that because of having to constantly cover herself in make-up and lipstick, how different the modern world is compared to the 1940s, not to mention the other kids teasing her; she'll never be a part of the world of color and brightness and chooses to remain in Grayworld with her friends though she still offers to help Tommy and Ben escape from there by creating another lipstick window. Her final words to them are "Don't forget me!" before the hole closes up separating both worlds from one another. But the terrifying ending result is Tommy, Ben, and the other kids of 1997 are eventually sent back to Grayworld by Mr. Chameleon so it's possible that they're all imprisoned in the classroom forever (though Thalia could presumably draw yet another lipstick window for them).
Thalia is considered to be a very beautiful girl due to the fact that most of the kids thought she was a complete stuck-up snob. Even when covering herself in thick make-up, she appears to be around Tommy Frazier's age, 12. She has long blonde hair and gray eyes. She wears blue contact lenses to hide her gray eyes. Thalia is tall (taller compared to Tommy who is really short by most standards) and thin. She also has a uncanny resemblance to Ben. Both looked so much alike that they were thought of as siblings in her gray form even though she didn't have any color at all.
Despite her imprisonment for 50 years and being bullied by the other kids of 1997, Thalia is considered to be a very kind person, evidenced by the fact that she attempts to save her friends in Grayworld. She also is friendly to both Tommy and Ben, actually betting that Tommy wouldn't get lost when he went to fetch the red buckets of paint she requests at the beginning of the book, and eventually helps Tommy and Ben escape Grayworld in the end. Despite this, she has a bit of a temper, evident when the main bully Greta steals her lipstick and she nearly goes berserk trying to get it back. She also laughs at most of Ben's lame jokes and appears to be a truthful, honest person as she reveals her true identity to both boys as well as how she escaped.