Tadashi is related to Yuki as a cousin and he is also the grandson of Masa and Takeo.
Yuki is very close to Masa and would likely spend every waking hour with his maternal grandfather if he could. As it is, he and his mother spend most of the summers there and he remembers those times very fondly. Masa has a difficult time understanding Yuki, however, particularly after her mother's death. Yet despite their difficulties it is through Masa that Yuki maintains her links to the family.
Takeo and Yuki's relationship is not as well developed but it is also clear to Yuki and the reader that she cares very deeply for her grandfather in a similar way that she cares about Masa.