The main character is Rokuhara Satoru. The ghost that always hangs around him is Kako.
Since he was a child, Rokuhara Satoru has had the ability to see ghosts. The ghosts of murdered victims point to valuable clues that help Rokuhara Satoru solve murder cases. After the cases are solved, ghosts usually pass on and disappear.
The exception are the vengeful ghosts. These are violent ghosts that do not pass on even after the case is solved. Two of them tried to kill Rokuhara in chapter 1-2. Kako, apparently, has the power to purify vengeful ghosts.
It goes without saying that the two of them became partners. Rokuhara solves the mysteries and Kako protects him from getting killed by vengeful ghosts.