He appears as a giant humanoid black cat. He has green eyes, and a yellow star-shaped symbol in his forehead, owing to his name. He wears wears pince-nez glasses, and a yellow haori with pink cat head print over a dark green kimono, which in turn is worn over a white han juban.
When Tamako had to move back to her home in Kyoukai, after her husband passed away, she took all the furniture from her human home, and Kuroboshi ended up stuck in a closet as all those furniture were put in there.
Wishing to retire, he asked Rinne to train his grandson Kuroboshi III to become Tamako's new Black Cat contractor. But since his grandson hardly makes any progress in overcoming his phobia, he won't be able to retire soon.
Once, he and Tamako resorted to manoeuvre in hope of forcing his grandson to overcome his phobia, but to no avail.
Although he hopes not having to serve her anymore, he does not show much animosity toward her.
Since his grandson hardly makes any progress in overcoming his ghost phobia, he feels disappointments in him. He nevertheless feels concerned when he apparently ran away on two occasions.
- "She hasn't changed" (まるでお変わりない) - Chapter 175, speaking about Tamako
- His name means "black star".
- His voice actor voiced the Tomobiki High School Principal in Urusei Yatsura.