In the information already provided for us, I have a couple reasons why I suspect that. Kaoru and Maki, Maki from First Comes Love, Then Comes Marriage. I suspect that she is the mother of Kaoru. the dad being Tetsuya, which is already married to Maki.
First Reason, is obviously the hair, and how Kaoru and Maki look so similar to each other. Another reason is on Chapter 35, when Kaoru says that his dad is a terrible cook. which I think is true, because in the other manga, on Chapter 2 I believe, it showed him just making some toast for breakfast. later in that chapter it also said that Maki made a really good dinner. So, like mother like son, Kaoru can cook well to. A third reason, is (idk if this really counts, but ill add it in anyways) when Tetsuya says to Maki, (P.S Kaoru wasnt born yet) that they should die together, and Maki doubted that shell live longer than Tetsuya. Hows this for my first theory? I'll change this when I find more evidence, but I wanna here you guys thoughts on this. But thats just a theory... AN ANIME THEORY