It's her goofy inner self and the whole attitude at home that makes her charming. While the girls all try to look as good as possible for Sou, Sumire openly shows her bad sides and expresses her horniness quite frankly. And yet she's still acting like a responsible older sister where it counts.
Honestly, if I had to pick a personal guard tag team for myself, then Kanaria and Sumire would be my preferred choice.