So the promise was writing a book about them living together, since Otone died after severe carbon monoxide poisoning while trying to save Tsugumi precious picture book (don't think she was thinking right, but also it was kinda like her real careless character) so she couldn't pass on to afterlife, it's what held her back...
In a selfish way, it's good she stayed back as a ghost that helped Tsugumi to not hold back, but on the other hand only Hiyoko could see her for 10 years, that must be a horrible and a lonely thing even for a cheerful clutz Otone really were.
Rest in peace Otone, you were an exceptional person that brought out a shy personality even a little bit to move on instead succumbing in expressionless life.
Thanks for scanlation!