I assume your character has no visible physical signs of demonic heritage - that would make the people around him wary of him and create more problems, isolating him from those he seeks to cultivate.
Internal dialogue can be very effective. He has been brought up to identify as human - possibly because he couldn’t be accepted by the demons as his human blood makes him weak. The alter ego will have a name and complete identity - who he might have been in this other society had his mother, for example, not been human.
While he wants the strength of his demon self, he maintains control in what he might see as an extreme case of an id. He must remain in control, but the reins of Plato’s horseman will slip on occasion.
I would choose to make the severity of the slip make regaining control initially more difficult, but once regained he will be in control that much longer. He has let the demon out to play and it is satisfied.
One problem he might encounter, since he liked having this power, he might start enjoying the ride. No control over his actions can be freeing, though the demon has wrought evil, he has not. He gets to be the angel on the demon’s shoulder, telling him enough is enough - time to relinquish control.
I had a mare who used to bolt every time I rode her - until I galloped her first. I would choose when and she became content to wait, confident that the fun was coming. Her reward was the equine equivalent of flight.
Your character can negotiate with his darker self some such compromise. The dichotomy will exist until death and he can persuade this intelligent other that as long as they are perceived to be him and a normal human, both human and demon are safe. Should the demon self go too far and reveal this dual nature to others and they survive (reminding the demon that massacres are messy and noticeable) that their own survival can be cast in doubt.
Robert sat in the restaurant, his lunch half forgotten, surrounded by
people like him - almost. He enjoyed this quiet time alone but not
alone. Never alone. Returning his attention to his meal, he felt
Shrateg stir and focus on the blonde two tables away. Her skin was
perfect, but how his darker self ached to change that.
‘Not a good time. Remember our agreement? Nothing that can be
witnessed. Nothing that will put us both in danger.’ He thought,
waiting for the response.
‘Never a good time. You only agree to an hour - not enough. You are
weak. We are not you, we are me.’
‘We’ll go to the park later. Beautiful day. People will be there. Just
no kids this time. Nothing to get us hunted.’
‘You fear them - I do not. They are weak, I am strong. I will keep you
alive if you follow me.’
The waitress approached with the bill, which he paid - gave her a
decent tip, nothing she would remember.
‘That woman smells of fear - I can take now and you can share the fun.
I know you like parts of it. I feel you as you feel me. You are no
He couldn’t deny that some aspects of Shrateg were wonderful. He loved
the sheer power of his darker self, his confidence and the clear
superiority he felt. Shrateg made him more than these others, much as
he craved their company.
He felt Shrateg’s joy and saw the blonde was approaching him, seemed
to want to join him. Hell, why not? He nodded greeting and she sat
“The food here is pretty good. I see you didn’t finish. Would you mind
donating that to the homeless?”
“That is a wonderful idea. I’m Bob.” Leave this to me, okay? Nice
girls are reported missing and should return home.
“Mary. Who’s your friend?”
Edit - It has been years since I last read Plato, so pardon any slips.
Plato describes the human personality as being composed of two main parts. Being a poet by nature, he used the metaphor of the horse to encapsulate passions, fears, desires, urges and needs. The rider, Plato’s horseman, is logic, reason and impulse control. The rider is the higher self, but is weak without the horse. One must not completely overshadow the other, but work in harmony.