Keiji is shown to be pretty casual and level-headed during the Death Game, though he comes off as shady to some of the participants. He usually refers to himself as "the friendly policeman" in an effort to establish himself as trustworthy and makes a pretense of being less cunning than he really is. 

He's observant and canny and is quite good at reading other people and knows when to speak up and when to leave them be. 

He's also encouraging and gentle with the younger participants, making it his job to at least keep watch over Gin.

To go more in-depth on his voice:

Keiji is a difficult one, for sure. His voice should be on the lower side, but not all the way down in the basement. He usually comes across as easygoing/laid-back, the kind of guy who would lean back in a chair with his hands folded behind his head as he spouts some nonchalant comment. And he’s definitely the teasing type.

However, when he gets serious, you can feel the shift in him. The tone of his voice hardens, sometimes taking on a grim or defeated undertone, depending on the context. He has no romantic feelings for Sara or vice versa and you can fight me (NKS director) on this. His teasing is more of a fatherly-type; y’know, guys who embarrass their kids by talking about how they’re so ‘cute’? That’s his vibe. Throughout the game, he develops a protective (yet encouraging) attitude towards his friends, especially Sara and Gin. He looks out for them while at the same time recognizing they are both very capable people in their own ways.

//spoilers// Keiji seems untrustworthy, or perhaps too good to be true, in the beginning. Later, it’s revealed to us that he has been battling his own demons for years now. He doesn’t say much about himself, even to Sara; not because he doesn’t trust the people around him, but because he does not trust himself. I don’t think it would be wrong to say that he may despise himself, but he hardly ever shows it. When he does show it, he gets this look on his face, like he’s cursed to be haunted forever… So ofc his tone would shift even further during those moments.

A very good comparison/example would be Yuichi Nakamura’s voice. If you're interested in looking into his work, some good examples may be Yoshiki Kishinuma from Corpse Party, Guren Ichinose from Owari no Seraph, or Kyohei Kadota from Durarara!