Harue Mizukoshi is childhood friend of Seiichi Mizukoshi. She became his wife later in the series. 

Harue shows up only on wedding photo, that Mizukoshi sent Nana K., when Nana K. thought that Takumi will never marry her, so she needs to break up with and find real happiness.

Behind the scences

Harue one of ancillary characters, so there is not much information known about her.