Responses
Well, I honestly thought that the Yakuza dude is the father. It's already given, plus ririko said that she was rejected and it was the yakuza guy that rejected her. (the yakuza dude literally said that she goes on and on, begging for him to comeback and whatever) So, at this point - the yakuza is the one that makes sense as the father.
Sentarou is just the one (most) reliable dude she'd met. Given by the past comment of your topic.
so does that last chapter confirm that sentarou is the father???