Short spoiler: We still won't reach the truth, bite-girl returns, Luna's full name is (染谷月)Someya Luna.
Long spoiler: Naoto went for the suck, but as the laws of love-comedy protagonists calls for, he doesn't actually know how to give Luna a hickey of his own. Luna, ever spoiling the MC, interprets the whole thing as him doing his old habit of diving to her chest whenever a lightning strike scared him when they were kids. Naoto separates, and Luna asks if perhaps he's homesick. Naoto resolves to stop being childish and went to sleep without taking a shower, while Luna took a shower and resolves to be like a mother to Naoto just to spoil him even harder, the mystery of the mark was never solved.
MC is downed with a fever, as one does after being exposed to anime/manga rain. This made Luna concerned, and declared that he should take it easy and rely on her as if she's his mom. Luna heads off to a dorasuto(that's drug store) and literally on her way out, encounters Kawashina. Kawashina was worried and was there to visit the sick MC, conveniently bringing some med supplies, and she introduces herself to Luna. She then asks if Luna is in a relationship with Naoto, but a flustered and blushing Luna denies it. Luna heads over to another room, and Kawashina continues to nurse the sick Naoto.
While wiping down Naoto's sweat, she couldn't resist and lightly bit his stomach. She is snapped back to reality by Luna saying that the rice porridge is done, and asks her to taste it. As Kawashina leaves at noon, relieved knowing that Naoto is not in a relationship and that she still has a chance, she was stopped by Luna. Luna tells her to call her 'Luna', because she's not used to being called by her family name, Someya. This triggered a quick flashback to the bite of chapter 5, and it turned out that Luna was nearby when it happened. The chapter ends with Kawashina learning that Someya Luna and Naoto's "Luna-nee" is the same person.