I originally thought this too, but the guy promised to marry her, and I don't think MC would forget that. Maybe it was a man connected to MC's Yakuza life somehow, or maybe MCs bio sister, since MC doesn't know his parents... Who knows. It sounds like author plans on making some connection tho
I think they mentioned the baby dad saying he only wanted some fun while he was deployed? So he could be a military guy or traveling for work and decided to fuck around with local girls, but MC (mom) probably thought he wanted serious commitment before the relationship started.
I'm questioning if that his kid? But I don't think he be that much of a scumbag to forget her right?