Short spoiler : Still no truth, Ryuuji is not Kawashina's brother, Naoto's synchro rate with KanoKari's Kazuya is getting unusually high
Long spoiler : Turned out, Ryuuji's a former co-worker of Luna's from her old part-time job. Hiba took notice because of his flashy 'player' look. Ryuuji being pushy, made Luna exchange contacts and then left. Luna mentioned that he never changed, then Naoto asks if they were close. Luna says Ryuuji's more like a 'younger brother', which Naoto took notice. Luna mentions that it's getting late, and that they should head off to their Ryokan to check in. Kawashina sees Naoto staring off into the distance, but that's just him being bothered by what just happened.
At the Ryokan, they encountered Ryuuji again. He offered to carry Kawashina's bag, swiping it off her while asking. Ryuuji noticed Naoto, and Naoto introduced himself. Ryuuji seemingly exuding some sort of pressure on Naoto until he mentioned being cousins with Luna. Ryuuji introduced himself, relieved upon knowing that Naoto is just Luna's cousin, and asked if they're on family vacation or something. Naoto answered no, and told him that he's actually staying at Luna's place, which surprised Ryuuji.
Ryuuji then teased Naoto in the most sus gyaru-o way possible, he asked Naoto if he banged Luna yet.
Ryuuji laughs it off being relatives, there's no way that could happen, and that the joke was too much for the virgin Naoto. Luna asks what's taking Naoto so long, and Ryuuji approached the girls a little too close for Naoto's taste, which puts Naoto in full on-guard mode.
Some time pass and yet they still continuously encounter Ryuuji. Ryuuji and Luna are then seen chatting while a sulking Naoto mutters how much fun the two are having. Luna noticed Naoto, and mentioned the place has ping-pong tables and that they should play. This reminds Ryuuji of the time he and Luna played it at their old workplace, and the two reminisce on their own. Naoto, going in a dark place mentally, thinking that it doesn't necessarily have to be him to make Luna happy, sulks hard and rejects Luna's invitation to play ping-pong.
Naoto says he'll just get in the way, and that Luna and Ryuuji should just go by themselves. Luna playfully asked Naoto if this got him jealous.
Luna was mid-way telling him to not worry about it, but Naoto snapped and yelled at her. Saying that for Luna, as long as she was having fun, anyone would do and it doesn't have to be him. Naoto immediately noticed his behavior, and begins to explain himself. His outburst clearly affected Luna, asking him why he had to say that, telling all she wanted to do was to 'Have fun with Nao-kun', and Luna runs off.
Kawashina and Hiba, fresh out the baths, finds Naoto. Kawashina asks him where Luna was, Hiba then mentions the two were with that other guy earlier, with Kawashina then asking if Naoto got separated from them, dealing Emotional Damage. Naoto then mentions how Luna looked like she was having fun with Ryuuji, and that he'd just get in their way.
The two girls laugh, Naoto asks why they were laughing. Kawashina then mentions it's because of how cute Naoto looked, implying that Naoto's showing the same face he does when he's sulking. Hiba then gave him an advice, think more about exactly who Luna wanted to have fun with. Naoto still not out of it, Hiba tells him to just get in the baths and cool his head while he's at it.
On his way, Naoto finds the old manager having trouble changing the sign for the baths. She told him that it was due time to change the sign from woman's bath to men's. Naoto offered to change the sign for her, but was told she haven't checked if there's still someone in the bath. She told Naoto that there's only three women customers in the Ryokan right now. Naoto deduced that since two already went, that only leaves Luna. Thinking that Luna's with Ryuuji, he convinced the manager and got in the bath anyway.
Taking a dip, he thinks about Luna and pulls another NTR-bait imagination spot. He imagines Ryuuji having his way with Luna, and we're treated to more black boxes. The more he thinks of Luna, the more he baits himself. As he snaps out of his imagination, he hears a noise, thinking someone else got in first. Chapter ends as as he looks towards the noise, and sees Luna.