CAS or Cassie was a kid who was stolen from home by GGY which is why she has no light in her eyes. Since Cassie is still a child, and, so was GGY, they became friends, and if GGY would go missing, Cassie will immediately go looking for him because he's the only 'family' she has left...but, he was the one that stole him from her real family. As they began to form more of a connection, Cassie will get a codename just like GGY and that would be 'CAS'.
(Extra information for CAS: Instead of the V.A.N.N.I mask being handed to her by a security bot(idk), it was handed to her once she got her codename 'CAS' by GGY. This was handed to her due to part of GGY's plan to join him in being controlled by GlitchTrap.)

GGY or Gregory is an unstable kid who murdered his Psychologists, who was surprisingly also Vanessa's Psychologists as well. GGY is a rather manipulative person and could easily trick people due to him being a kid, but would sometimes have trouble when trying to trick other kids his age. The reason why GGY is like this is because of GlitchTrap, this caused him to have a split personality named 'Dr. Rabit', this personality has taken over completely which would cause the real Gregory to be completely lost.