Kouji-kun had already demonstrated his own evolution to being a Man Of Culture (attraction to older woman), but even before the close, Chizuru-san has attained the level of "Sexy Onee-San". Figuratively speaking, Kouji has virtually no chance of victory ... and (to borrow the song lyrics) "That's The Way (Uh-Huh, Uh-Huh) I Like It (Uh-Huh, Uh-Huh)!"
A better arrangement for everyone; she (presumably) stop seeing his father, the son spends all his time with her, and she gets to keep her taboo pleasure going. I wouldn't mind seeing a sequel that goes into their relationship as he grows up and how that could change, because it could be funny if she would become his girlfriend and his dad learns that, it could lead to some interesting interactions.
A fitting title...Secrets and stuff.
And well hey, I'd say taking her away from your dad is certainly...One way to solve the problem.
That ending though. "How it feels to go bad", eh? Somehow I found that phrase rather intriguing.
Well I understand not tagging this as cheating since there were no scene of Kouji's dad and Chizuru having sex. But I was wondering if it's considered cheating if a mistress have sex with another man...??? I'm mindfucked right now...
Uh, ... well, unless there's a continuation to this, it's quite possible that Chizuru's no longer involved "in that way" with Kouji's dad, unless that's being left to the readers' imaginations.
EDIT: Still, "Sexy Onee-San" is Win (Kouji, you Lucky Bastard) ..... "DAYUM!"
EDIT (again) according to the promotional volume for the upcoming tank "Secret x Secret", Chizuru-san is *koff-koff* "doing it' with BOTH Kouji and his father (separately, not together) ..... "BUT IT'S WRONG!" (maybe that will get fixed when the tank does come out).