So, I just finished watching the anime, and then went back and read the manga from the start. I've got to say, that I'm now officially on board the Nana bandwagon, and hope like hell that this story sees its way through to the end. Good luck to Ai Yazawa, and good luck to all the fans who eagerly await answers. What a great story.
And since there seems to be a lot of speculation as to who the two kids belong to, I'll post my theory here.
I believe that one of the children is Nana Osaki's.
I'll refer to Nana Osaki as Nana, and Nana Komatsu as Hachi, for clarity's sake.
At first, I thought that Satsuki had to be Nana's daughter, and that Ren had to be Hachi's son, based on merely physical appearance. Ren has the same hair color as Hachi, and Satsuki vaguely resembles Nana. But once I thought about it further, it seemed almost too obvious.
Here's some reasons why the first scenario might not add up. Ren seems to be easily a year or two older than Satsuki. Hachi was pregnant for a while with her baby before Ren died. But up to the point that Ren dies, we're never given any indication that Hachi is close to her due date. When Ren died, Nana hadn't seen Ren for about a month. I know there was mention of Nana forgetting to take birth control pills a few times while Ren was still around, but even if he did get her pregnant, Nana's child wouldn't be that much younger than Hachi's. Less than a year in age difference. Yet Satsuki seems considerably younger than little Ren. There is also a scene in the manga where Hachi and Satsuki are out shopping and talking about Valentine's chocolates. Satsuki finally confesses to Hachi that she wants to buy chocolates for Shin. After that, they go to Jun's place. When Hachi tells Jun that Satsuki wants to buy chocolates for Shin, Jun replies that Satsuki has definitely inherited Hachi's genes/DNA. If Satsuki wasn't really Hachi's daughter, then why would Jun say that? After Jun mentions that, Hachi's reaction is that of a mother worried that her daughter has inherited her taste in men, not the reaction of someone who's hiding Satsuki's true identity from Jun. Its because of this that I think little Ren may be the child of Nana and Ren Honjo. But not from the Tokyo era. I think Nana may have got pregnant with Ren's child before he left for Tokyo. Ren had been in Tokyo about two years before Nana decided to go. Right before he tells her he's going to Tokyo, Nana mentions wanting to go take her pill because she can't afford to have a child at the moment. But rather than go take it, her and Ren start fooling around and then he tells her he's leaving. After that, a year and nine months pass before Nana mentions buying a ticket for Tokyo. I think its possible that Nana gave birth to Ren and then gave him up to an orphanage. If you remember when Nana was lecturing Hachi about mothers not taking having a child seriously, my first instinct was that she felt so strongly about it because her mother had abandoned her. But after this possibility, I think that maybe she felt strongly about it because she gave up her own child back in her hometown, and thus followed in her mother's footsteps. Maybe down the line, Hachi finds out about Nana and Ren's child and adopts him from the orphanage. I could definitely see her doing that to have one last thing to keep her closer to Nana. And I wouldn't see Takumi putting up a fight about it because he and Ren were close. It's mentioned several times that Ren is very independent and will often wander off alone. Could this be mirroring his parents' personalities, and also because he may not have had a lot of affection early on if he spent the first couple of years of his life in an orphanage? Nana also was expelled from high school, so that wouldn't interfere with keeping having a child a secret. Although, I don't recall how long BLAST stayed together after Ren left, so that may pose a problem with this theory, as I can't see Nobuo and Yasu being completely in the dark about this if the band was still together while Nana was pregnant. There is a narrative line by Hachi, in the chapter where, around Christmas, she receives the envelope with pictures of a long-haired Nana singing. Hachi says, "Every year when Chrismas gets closer, I pray to the sky for a miracle. Not for angels or Santa Claus. But for the night when Nana and Ren can meet again. At this point in time, Ren Honjo is already dead. Could Hachi be referring to little Ren? Or could she just merely be praying for the impossible, or for Nana and Ren Honjo to meet again in the afterlife?
Although, little Ren being independent could be a product of Takumi's personality, and he could be Hachi's/Takumi's son after all. Nana could have got pregnant before Ren died, and just hasn't realized it yet, and Satsuki could be slightly younger and just smaller than Ren. But then that contradicts the DNA comment that Jun made. Ugh, I just wish I could find out for sure. Either way, I still think one of them is Nana's/Ren's child.
I'm not sure if this is how the story will end up or not. But its the theory I'm going with at the moment. Thoughts?
Let's all hope for the best for Ai Yazawa, and a conclusion to an amazing story.