"Jax, look. I know we don't get along, but I think I might have found out who the sixteenth student, lying hidden somewhere in this school is. The one they call the Ultimate Despair."
I have a theory that whenever Reigen invites Mob out to eat, he takes him somewhere where he doesn't have to use a spoon: ramen, takoyaki, hamburgers, steamed buns. At some point, he realized how insecure that made Mob feel and decided there was no need for him to feel that way.
System who has emotional amnesia instead of the expected separated experience memories and so has a shared memory except for stressful emotions if that makes any sense