Slow Step (1991)

Based on the Adachi manga of the same name. The story revolves around a romantic triangle formed by the girl Minatsu Nakazato, a young student and softball player; Shu Akiba, a schoolmate; and Naoto Kadomatsu, an up-and-coming boxer. In reality the three-way relationship is more like three-and-a-half, considering that Naoto is in love with Maria... actually Minatsu herself, who'd once disguised herself to escape some evildoers.