Shinobu Sensui was once a rei kai detective (spirit detective), who had amazing judgement and powers since he was a child. But on one case he came upon a group of humans torturing innocent youkai (demons). Sakyo was in this group. When seeing that, he tried to protect the youkai, but failed. After that, he gathered a group of seven people with supernatural powers like his. Sensui planned to make a hole in the barrier between Ningenki (human world) and Makai. Although the only reason he planned it was to get Yusuke's attention. For he still had other plans.

Although he may not look it, Sensui's powers are very strong and he is rated as an S-class demon. He also has seven different personalities in his one body that come out in different situations. (Though you can never tell...)

Since his powers exceed Yusuke he kills Yusuke. Hiei, Kurama, and Kuwabara had no chance against him either. He finally beats Sensui when Yusuke is reborn because of his youkai blood and gains S-class powers.