bio:The story all begins when Yusuke Urameshi, a crude, rambunctious, selfish, seemingly uncaring young man with an even less caring mother and a reputation for being a toughest fighter in town (and he is, too) did something so unexpected that no one, not even the Spirit World [Reikai] had been expecting it.
He did something genuinely good...and ended up roadkill for it.
Because of the unusual circumstances, Yusuke is given an opportunity to be revived. He is reluctant at first...but after he learns just how much me meant to others, he agrees to try. It's a rough, brutal ordeal and in the end relies on both the goodness hidden deep in his tough exterior and the feelings others felt for him...but he ultimately succeeds and rejoins the world of the living.
In exchange for his new lease on life, Yusuke is appointed the Living World's [Ningenkai's] Spirit Detective. You see, being a resurrected human has its side effects. Having been a ghost once, Yusuke has become spiritually sensitive, giving him the ability to see intruders to the Living World [Ningenkai] otherwise invisible to ordinary humans: ghosts and demons [youkai] and the like. He also now has a special technique that gives him a much more potent weapon than sheer physical force: his Spirit Gun [Rei Gun], a concentrated bolt composed of the sum of his spirit energy. Originally, he's limited to using the technique once a day, but in the course of his cases, he receives special training to further boost his spiritual abilities and give him advanced techniques like the Shotgun (a multi-shot Spirit Gun burst). Also, he receives various devices from the Spirit World [Reikai] to assist in his missions (such as a spyglass and a communicator).
Later on it is stated in the series (just before the Saga of the Three Kings, and several times during it) that King Yama terminated the Spirit Detective Department, thus making Yusuke an ex-spirit detective. Also, his Demon Energy is that of an S-class Demon (not suprising considering Raizen is S-Class).