It will never stop being funny that all the tropes of hidden power levels, auras, training arcs, sudden power-ups, etc. are all there from Shounen manga, but just with horni replacing fighting ability.
I don't know exactly but Sumire looks good in any dress. Her being in Formal may have made that face attractive. Also her 'Snake-eye' face is special toxin which has exotic and dangerous vibes. Last time it happened when Girls were at interview at Satou home. There she felt less dangerous but allure was there. When she has angst with other women, she looks alluring.