The Prince is Sina's brother in "Prison Princess". He starts off in her memories as a weak and cowardly prince. She was always able to beat her older brother in a fight and he never forgot that. He ends up melding himself with the demon Overlord and becomes one with him and is the reason his sister is being held captive. He plans to kill her in front of their father but decides to show her who the strongest is now by beating her to the brink of death. 

The first line is when he is himself originally, the next 2 are once he is revealed from the Overlords form, and the last 3 are when he is fully in overlord form. 

Ideally his voice should be a bit higher and weak sounding in parts, but also sound menacing in other parts as well. If it could be a bit deeper when he's in overlord form that would be ideal as well.