Katsuragi-san is the "class queen" that he crushes on from afar (she shows up for the first time in chapter 4) - and she also happens to be his childhood friend.
But as usual, he "got conscious of" her around middle school and put distance between them, and she went on to be super cute and popular and he became the loner otaku. He was going to confess to her once he reached floor 2, (obviously) chickened out, and we see her once more like a dozen chapters later where she is watching him sleeping in class covered in bandages.
And now she's shown up here, wherein my guess is she'll create a misunderstanding as he's in a "girly store" (I'm assuming searching for stuff for his two comrades), and she'll think he's got a 'special lady' in his life and ....I guess we'll see.