Ep. 8: Koyomi faces Karen's unexpected request involving her sister's clothes and a tooth-brushing challenge.

Koyomi is studying when Karen arrives wearing Tsukihi's clothes, acting more feminine than usual. She asks Koyomi to introduce her to Suruga, who is her friend, but he refuses out of fear that she might pursue his sister. Karen then challenges Koyomi to a contest, promising to grant his request if he loses. Koyomi selects the terms: he will brush her teeth for five minutes without resistance, a condition she accepts. As he complies, both participants become increasingly aroused. Tsukihi interrupts the scene, demanding an explanation. Although Koyomi accepts defeat and fears Tsukihi's punishment, Karen asks him to resume their activity after she leaves. The source details this specific interaction without providing further context on the outcome or subsequent events.

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