It would be cold to say that Ami's otousan was heartless. How could he leave his family, even after a divorce? But as it often is, men become confused. Sometimes they do not know what to do in time of turmoil such as the death of a loved one or such a thing as separation. We cannot assume that Mizuno-san did not love his daughter...his yearly paintings that were sent giftwrapped and filled with love show his feelings...but can he not face her? Once upon a time he taught her to play chess and how to swim...we often see her at the club that she can sneek into through her father's membership. If her otousan were around, would there be more love for Ami?
And what to say of her mother? The doctor whom Ami looks up to everday in her life? She, as Ami's father, is never seen, but heard of. All that is known of her mother is that she is the one who has raised and loved Ami for the past few years of her life. She is a pediatrician, which is Ami's lifelong dream. The two share much in common, which is just as well.
At one time I actually saw a battle on who was really the better parent. Although I admire both, I do agree that Ami's okaasan is the "better" parent. But Her father isn't so bad in his own special way...