Ryuk barely reacted to the cat. A small twitch at the sudden development, maybe, but that was about it. He just stared for a moment as it launched itself at them.

And then he remembered he was pretty much human. He couldn't walk through walls and a lot of people had stared at him today. So...the cat could probably hurt him. And that was why he took a step away from Mikami, hoping the cat would go to the human first. That way, Mikami could fight it or just distract it from Ryuk until it had eaten it's fill or whatever it was after.