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"[Character] has the brain cell."
Yes, they do have the brain cell, but unfortunately, the singular brain cell that they possess is an astrocyte, the most common type of brain cell, and not a neuron.
Astrocytes play a role in the structural support of neurons and maintaining a healthy balance of neurotransmitters, so that character's singular brain cell is simply there for support. It isn't actually there to do any thinking.