Here's what we know:
- Reiichi (on his birthday) texts Kaoru and says that he's going to be out of town for an extra couple of days.
- Kaoru calls Uta, who had been planning on staying over at Kuro's, and tearfully says she's lonely; Uta says she'll come home after work.
- Kaoru goes out in the rain to get some groceries to make dinner; she sees a text from Reiichi and calls him, only to realize that Reiichi, who has just told her a few hours ago that he'll be out of town for a few more days, is walking with Risako on the street below.
She slips on the stairs and we later learn that she hits her head, fractures her leg (or ankle), and is hospitalized. (Chapter 10).
In the hospital Reiichi starts to apologize for "the things happened during the accident," [Looks like a word missing there] when he realizes that Kaoru doesn't remember anything (as the doctor tells him, this memory loss turns out to be temporary).
Later in the hospital, he gets a phone call, obviously from Risako to whom he says:
"Yeah, she's fine."
"Sorry, I made you worried."
"No, it's not your fault."
"Anyway, I'll tell you the details later."
"See you soon, bye."
He then slams his phone against the wall, looks out morosely, and in a big black panel thinks, "What am I doing. . ." (Chap. 11)
Risako comes to the hospital after visiting hours and talks to Uta, giving her a gift for Kaoru along with Karou's cellphone that she says she picked up at the scene of the accident. As Risako leaves, she thinks about Uta's affect, and thinks, "It wouldn't seem odd even if I were her sister at this moment."
The part with Risako could just be her thinking that Uta is cute and seems reliable, and things might have fallen out so that Risako had married Reiichi rather than Kaoru.
Reiichi's actions and later reactions, though, are much more difficult to explain away. It's true, even if there were an innocent explanation for his presence there with Risako, he might well gloss over it if he thought Kaoru didn't remember anything anyway--why bother with an excuse if you don't need one?
But the whole "What am I doing" thing looks bad. What is he doing, in relation to Risako? If the mangaka knows, we ain't being told.
If you claim to be out of town on a business trip for "a few extra days" on your birthday and later that day your spouse spots you with another person, one you used to date, well, . . .