A Spirit Detective (霊界探偵, Reikai Tantei, lit. "Spirit World Detective") is an agent of the spirit world hunting down renegade demons who threaten peace on Earth. Koenma is the one who grants this position.
Normally, the condition to be a Spirit Detective is to either have Psychic abilities awakened or have great potential. This Spirit Detective is hired on his/her prime, either as a teenager (14 years old) or a college student (17~18 years onwards). The Spirit Detective commonly has the help of a Spirit which works as a liaison and tactical support.
All Spirit Detectives have the ability to fire spirit energy in a blast, that can be channeled between their fingers (like Yusuke's Spirit Gun), hands (as a spirit ball by Kuroko) or by their legs (as in the case of Sensui).
If the situation is too extreme, Koenma grants the Spirit Detective temporary allies that can help in times of need, but the SDF never intervenes in this situations unless the Spirit Detective services are no longer needed (because of an emergency or the Psychic human is not strong enough).
Such rank has been passed among three known persons during the series. This position can be regained in times of need, like when Kuroko Sato was warned by Ayame, on behalf of King Enma to eliminate Yusuke Urameshi, to which she declined. Also, as the Spirit World governor rule changed, Yusuke reopened a food business where he secretly continued his Spirit Detective business, in order to better relations between humans and demons.
Known characters who were granted the title
This is the list of all the characters that have been a Spirit Detective in the Yu Yu Hakusho series.