I know it's the typical cliche "tomboy" and "girly girl" look, but something about their designs just feel right. Maybe it's the author's art style?
I think that the art is remarkable.
IMO the artist's secret is how they draw faces. Both protagonists are extremely emotive because the author does not rely on a standardized set of facial expressions but draws them from scratch in almost every situation. Note also how huge their heads are compared to their bodies, even for a manga, -- this gives the author even bigger canvas to register Ise and Shima's minute emotions.