Keiji is shown to be very quick to trust Sara once meeting her in the Death Game, Keiji often refers to himself as the "friendly policeman" throughout the game. In the main games, Keiji can act cold and harsh towards those who he suspects, whereas outside of the main games Keiji may act trusting and kind towards all of those entrapped in the Death Game with him unless they have done something which makes him suspect them. 

I imagine Keiji's voice to be low. He does not show too much extreme emotion in the game. He needs to sound caring while comforting Sara.