Yeah we don’t know everything yet. Steven was created to shoulder (and downright ignore) his mother’s beatings. But why was the third personality created? Both Marc and Steven are programmed to literally forget about him almost instantly. And we know this personality emerges to kill young adults. Could have been the personality during the cupcake truck chase. I think Steven had to turn to sand to force Marc to explore that trauma and we’ll finally see the third personality, Jake.
If anyone hasn’t read A Fractured Mind by Robert Oxnam, I highly recommend! Oscar Isaac read it in preparation for this role and you can clearly see he took a lot from it for his performance and production cues.