Koenma

Name: Koenma

Special Power: Pacifier

Anime Show: Ghost Fighter

Biography:

King Enma Jr., usually addressed as Koenma is the son of spirit realm ruler King Enma, and the one who takes responsibility of his father's work when he's away. Koenma usually appears as a toddler with a pacifier in his mouth, though he is said to be at least 700 years old. He can also transform into a young adult form, retaining his pacifier and the letters "Jr" on his forehead. Koenma acts as Yusuke's boss, assigning his cases. 

Keiko Yukimura

Name: Keiko Yukimura a.k.a Jenny

Special Power: Eugene love interest

Anime Show: Ghost Fighter

Biography:

Keiko Yukimura is Yusuke's childhood friend and classmate. She has known Yusuke for many years and acts as a parental figure, often trying to get him to attend his classes and behave, though usually to no avail. Yusuke attempts to keep his job as spirit detective a secret from Keiko though she finds out about it during his third case, as the Saint Beasts end up using her as bait to anger Yusuke.

Shigure

Name: Shigure

Special Power: Boomerang Circle

Anime Show: Ghost Fighter

Biography:

Shigure is a demon surgeon and Osteopath. As payment for his services, Shigure requires that his patient have an interesting life story. As well, he requires of them a part of that life, though in eccentric ways. While looking for his lost tear stone and the Ice Kingdom where he was born, Hiei approaches Shigure and hires him to implant the Evil Eye in his forehead.

Raizen

Name: Raizen

Special Power: Energy Blast

Anime Show: Ghost Fighter

Biography:

Raizen is one of the three kings of the Makai (in fact, he was considered the strongest of the three) and Yusuke's ancestor. 700 years before Yusuke is born, Raizen, then a young demon, comes to the human world looking for people to eat, and is massively injured in confrontations with human warriors.