I don't have a favorite character so I did expect Gigi to die and get betrayed by the Red Devil. After the episode, Thanksgiving, the list of suspects have been narrowed down a little.
The Red Devil and Boone are the children of Wes and Sophia. For me to identify who the Red Devil is, I look at their physical appearance from the Red Devils' parents. We now know that Wes is the father of Sophia's children. Both Wes and Sophia have dark hair and I assumed their hair color are dark brown. We know that Boone has dark brown hair color so his sister should have the same hair color too. Both the Red Devil and Boone were under Gigi's care and Gigi raised them. I personally don't believe that one of the twins are adopted when they were younger. Both of the twins were at the asylum with Gigi. This statement is proved when Boone said he grew up with weird people around him at the asylum.
The Red Devil and Boone don't know who their father is, they only know who their mother is which is Sophia Doyle. So, these are some characters that I do not think is the Red Devil:
Chanel - she is an Oberlin, which means she is not the baby in the bathtub.
The Chanels (#3 & #5) excluding Hester - they are not the baby in the bathtub. I assumed their families are pretty rich.
Grace - lived with her father, Wes, all her life. Is not the baby in the bathtub because she was born 9 months after the incident happened at Kappa in 1995.
Zayday - she mentioned that she lived with her grandmother as she is grew up motherless.
Pete - Since Boone is the other baby in the bathtub, we know for sure Pete isn't. So, Pete is out.
Wes - too old to be the baby in the bathtub.
The Dean - is not the killer because the other Red Devil is at least 20 years old.
SO, my main suspect is Hester. Although she did mentioned that she has a mother and father (her father died when she was little) in the 2nd episode, I think she made that up. Hester has dark brown hair and when she first arrived at Kappa house, she somehow wore a 90s outfit. Could possibly what Gigi would make her wear. Hester has a weird interest for death and even has knowledge about the history of KKT and how to dispose the body. Btw, does Hester really have a bad accident in the past that made her to wear neckbrace?