The way he talks to Rex like he's real. And how Rex always verbally abuses him and almost killed Trina. I feel like Robbie is very deeply psychologically damaged.
Cat's really messed up but something feels more stupid and/or childish than extremely mentally ill. Jade is very violent but not like she's got anything too deep. Sinjin is creepy but sometimes proves to have more sense than the others. (i.e The Slap Fight). Sikowitz is a good teacher with weird methods and who liked coconuts.