WHAT'S THE ONE EMOTION!?!? Is it happiness?

I think its love.

Spoiler(ithink)



Thats his care for his sister... Any other emotions are already gone...

Love for his little sister is the only one emotion left and it can be demon emotion. (≧∀≦)

Spoiler Darkangel is right.

The only emotion left was his feelings for his sister, which wasn't romantic love, but his feelings as a Big Brother, before the experiment.