My Sister and the Moonlit Dance (妹と月夜のダンス Imōto to Tsukiyo no Dansu) is the 2nd episode of the Kono Naka ni Hitori, Imouto ga Iru! anime series.
Shougo Mikadono chased the stalker to the rooftop, only to find out that she was Mei Sagara, a third year student who worked in the Sister Café, a magical girl-themed café. She was watching Shougo because she wished to know more about the life of the heir of a entrepreneur, herself having born in a business family herself.
Shougo skipped school to go search his family register, but the paperworks did not listed he having any sister. However, someone wearing hoodie approached Shougo and pointed a knife at him, demanding him to walk into a back alley. It was Ikusu Mizutani, a member of the Seiryu Association and initially introduced as Mister X, who explains that his father, Kumagorou Mikadono, had an illegitimate daughter after an affair, and it would be a scandal if Shogo were to marry this girl. Isuku also mentioned that Shougo's mother, Kanoko Mikoadono, had no knowledge or desire to meet her step daughter, limiting those who knew of the girl identity to her parents.
Having warned Shougo that his sister might have approached him secretly, Ikusu enrolled at the Miryuin academy as a male student to keep an eye on him. She also recommended Shougo having a DNA test to confirm his sister's identity.
At the student council dance party, after Shogo has a dance with Konoe, a confident Rinka Kunitachi, the student council vice president, challenges him to a dance, receiving quite an applause from the onlookers.
Shogo founda Miyabi outside, who was too shy to dance with anyone. They did had a dance under the moonlight, but the top of Miyabi's dress later becomes undone. Luckily, Konoe finds the two, and she helps fix Miyabi's dress, much to Shogo's relief.
After the dance, Shougo's sister called him again and revealed that she was right next to him during the ball.