Botan is the grim reaper, or guide to the underworld. She is charged with ferrying souls of the deceased to the underworld to face their final judgment. She also acts as Yusuke's assistant and trainer as he starts out his tenure as underworld detective. She teaches him how to use the various detective tools, communicates messages from Koenma and often guides him to different destinations pertaining to his cases. She will also join him in the field occasionally, ending up protecting Keiko from insect crazed humans, acting as team trainer in the Dark Tournament, and other various tasks.
Source from Wikipeida