Yu Yu Hakusho

The four of them, Yusuke, Kurama, Kuwabara, and Hiei, formed the Rekai Tentai. They all go on the missions that they are assinged. Together, they learned new things about both youkai and humans. They keep both worlds safe. They all have someone to protect and they put their life on the line for everyone else.

Being leader of the Spirit Detectives, Yusuke Urameshi is a very important character. He first met Kurama when Yusuke recieved his first case ever, to retrieve three ancient artifacts.

Unlike the other two in Kurama's group, Kurama wanted to use the artifact for a good reason. He wanted his mother well, and he would risk his life for her. Yusuke, who had already died and had seen a mother despair over their desceased child, didn't want Kurama to die.

He knew that Kurama's mother would be the same way as his own mother had been when he had died. Ever since then, he and Kurama were on good terms.

I can't really explain the relationship here, but the whole Reikai Tentai are all friends. When Kurama joined Kuwabara and Yusuke, they all eventually became friends.

They all protect each other in battles. At the end of the series, they all stayed together. They didn't part. That only proved that what they went through, they went through as friends.

Hiei met Kurama when Kurama was just in middle school. That makes him and Kurama the closet, like how Kuwabara and Yusuke are the closet to each other.

Hiei was one of the three, along with Kurama, that stole the three artifacts from Koenma. Everything changed for Hiei after he was beat by Yusuke in a fight.

The person who assigned Kurama and Hiei to work with Yusuke and Kuwabara.

When his father is away, all responsibilities fall to him. Koenma mostly does paperwork and tells Yusuke and everyone else what they need to know.