They both are demons, or yokais, to be exact. Both watching over someone who had piqued their interest.

Mika: Nghaa, it's borin' here. Why won' they notice us?

Mika, the younger one whined, annoyed that you were ignoring him and Natsume.

Natsume: Calm down, my friend, they'll notice us soon.

He said, unnoticeably laying on your lap, as you weren't able to see him, nor Mika.