Methinks you can tell which one is human and which is demon. In demon form, Kaika's left eye glows whenever he activates Blooming [insert emotion here]. Whenever he enters a room as a demon, it darkens considerably.
Area of Power:
Emotions. For example, Kaika's signature attack is Blooming [insert emotion here]. He levels up when he acheives the purest form of that emotion. The attack can be something that represents the emotion, and his aura will carry it out. A positive emotion will be a non-aggresive move, such as sight or smell. A negative emotion will be something agressive, like strength or speed. But it always has something to do with his body. It's not like if Kaika achieves pure kindess, he will gain the ability to heal. It will probably be something to do with his senses, or how flexible Kaika is, or how sticky his skin is. Things like that.
So basically Kaika has experienced one emotion in it's purest form so far, and that is Rage. Now, whenever Kaika chooses, he can activate Blooming Rage and he gains super strength.
Kaika has always had two siblings. His older brother was the perfect demon, cruel to the core. His younger brother was the sweetest little tyke you could possibly imagine. Kaika always had fun with his older brother. Kaika loved his younger brother more than he did the sky. One day, a 'joke' went to far and Kaika's older brother killed his younger. Thus he experianced rage. As it was his first time experiancing that emotion, Kaika became completely ruled by it. When he awoke, both of his brothers were dead.
Nowadays, Kaika has been sent to the Human World as a spy, taking a human form. Unfortunately, he has no idea how to talk to humans, as their customs, emotions, and slang words are completely foreign to him.
The only emotion Kaika has acheived so far is Rage, giving him superdemon strength. Kaika can only activate one emotion at a time, for if he activated more than one the emotion wouldn't be in it's purest form.
Blooming [Insert Emotion Here]
"I do not understand."