Koenma's powers are in his trademark pacifier. Actually, that pacifier gathers his pure rei ki and can be used as a powerful seal to capture dangerous and strong Youkai. This seal is called "Evil Sealing Ring". A funny note: Koenma always changes his looks when he goes to the Human World [Ningenkai] because he doesn't want people to make fun of him. He transforms into a very nice looking young man, but always keeps the pacifier. Koenma is actually hundreds of years old, but he looks like a 4-year-old kid.Sometimes Koenma doesn't just LOOK like a kid, he can act like one too... especially towards George. But as the series progresses, Koenma seems to become more serious. In the Sensui saga, we learn about his feelings of guilt towards Sensui. After all, Koenma gave him the orders for the mission that changed Sensui's whole life. Koenma decided to use the Evil Sealing Ring against Sensui, but it didn't quite work out. Sensui set things up so that Koenma would use some of the Ring's ki to save Tsukihito (the Gamemaster). Then, with Sunsui's superior saint-like ki, there wasn't a chance that he'd be captured. Also in the Sensui saga, he refused to obey his father's orders for the first time. He refused to return to Spirit World [Rei-kai], and followed Yusuke to the Demon World [Makai] to fight Sensui. In the last story about the Hell [Makai] rulers, Koenma secretly gave Kurama a location to train strong A-class Youkai, hoping that people from the Spirit World [Rei-kai] wouldn't find out.