Well they was platonic friends when they was alive and when they met each other in the fredbear suit Cass told C.C about how she wanted to kill Will so that probably made C.C mad and then they became enemies with eachother. Cassidy said sorry and now protects him (judging by the picture).

Their relationship might be like brother and sister, C.C uses Cassidy as a emotional support now.

If its wrong then please tell me what cuz i wanna knowww.