From Chibi Manga:

After her mother passes away, her father re-marries and due to his job he has to transfer, so Sachi decides to stay and live alone. She doesn’t feel as though she exists when she’s in class, and with continuous misfortunes, her feelings of wanting to live have become weakened… And at the present time she met the God of Death whose motto is "health first"!? The God of Death is Sachi’s distant relative and is living in human form a health education teacher, even attending her school and watching over her, but what is his real intention…!?