Spirit Detective - 霊界探偵編
 

I'm gotta make this really short here. This is just the start of the fighting part of the story, even its good too but nothing great compare to the story after.

1) Kurama and Hiei, Gouki

At the beginning Koenma gave Yusuke a case of catching this 3 demons as they stole some precious from Spirit world. But Yusuke underestimated their power. After a hard fight Yusuke killed Gouki, Kurama was actually using the precious mirror to save his human's mom life so they were on the same line, defeated Hiei.

 
 
 

2) Genkai Tournament

Yusuke is convinced by Botan to spend his three day weekend fighting in a tournament hosted by Master Genkai who is an old lady looking for an apprentice. Only Yusuke and Kuwabara was there and at the final, one of the fighter was actually a Demon, Rando and he was going to destroy everything and get power from Genkai. Yusuke finished him off, not so easily.

3) Four Saint Beasts ( 四聖獣 )

Now the story is getting a little more interesting. Yusuke and Kuwabara feel uncomfortable when they find people who seem to act like zombies everywhere around the city. Botan tells them that this is because they were summoned by the Demon world Whistle so they have to go the Maze Castle , home of the four Saint Beasts to break the whistle which is in possession of the leader of the Saint Beasts. Yusuke and Kuwabara are transported to outside of Maze Castle and Botan tries to protect the city from the zombies. Kurama and Hiei first join Yusuke's side for a battle, reason they do it was Koenma promised to reduce their crime if they help Yusuke out. The four Saint beasts was : Genbu defeated by Kurama, Byakko defeated by Kurabara, Seiryo defeated by Hiei and the Master Suzaku defeated by Yusuke.

The four saint beast name are actually from Chinese myth. but I found Japanese comic really love to use it.