SPOILER
I don't think it takes away from the overall quality of the manga at all, to have some questions left unanswered. To me, the issue of Rin's father was never really a focal point to begin with, and therefore not really vital to the plot. If that's the only thing that's unsatisfying about it, you might want to consider contemplating what this story is about.
To me, it was all about growing up and finding yourself. And the author delivered that in spades. It's okay to be disappointed with some things, but I would read this manga again because, in-spite of everything else, the female lead is strong-willed and determined and will not allow anything or anyone to hold her down or back for long. That in itself is what makes this manga so good, whereas with other manga's you will find female leads lacking in personality and strength of character. For this fact alone, I would reread it.
it'a an ok manga.. however I probably won't read this again... ending feels rushed and there's quite a few questions that was left unanswered ...like regarding Rin's father ...Miyu always tells Rin that she'll tell her but it got pushed back and so on and on and in the end no answer... very unsatisfying...