Otoko No Isshou

From Stiletto Heels:
Douzono Tsugumi, who works for a major electrical company, spends her vacation at her grandmother’s home in the country. Her grandmother soon passes away during hospitalization, and Tsugumi decides to continue living in her grandmother’s house. However, a mysterious man shows up holding the key to the rear house, and despite not fully comprehending the relationship he had with her grandmother, they begin living together.
Nominated for the 3rd Manga Taisho Award (2010).