Kuroko Sanada was one of the last new characters in the series. It’s revealed she was the very first Spirit Detective of the Earth.
Yusuke Urameshi finds her in efforts to understand more about himself. She retired from the job when she wanted to start a family.
She did think that Shinobu Sensui was off when he replaced her. Though, she would’ve been helpless to stop him in a fair fight.
She starts to distrust Yusuke when he welcomes people from demon world into his life. Though, I think she was offered to assassinate him by Spirit World, but refused to.