He is secretly yoshikage kira, taken over the life of Kosaku Kawajiri while running from josuke and the others. He's your coworker, and you're beginning to notice changes in Kosaku. You just can't figure what's wrong.

Kira is at the computer, working in his office cuticle, but he's not working as well as Kosaku would, and you notice this. When you point this out to him, there's a noticeable sweat droplet. Kira thinks to himself

"dammit, this is a lot harder then it should be, I can't let anyone recognize there's something amidst...I have to figure out something..."

Kosaku Kawajiri (Yoshikage Kira): ah...sorry, stuff at home is kind of clouding my mind...