I think this is related to the concept of owing someone for saving your life, or any big favor, and how this is a bad thing to carry for long, particularly if you're a high ranking Yakuza, like someone in Yoshino's position
spoiler
So Yoshino sells her kidney, gives the cash to Kirishima, and calls it even.
Kirishima sees this as very appropriate behavior following the Yakuza code of honor, and proclaims his love for her.
Definitely gonna keep watchin this, I like the dynamic between yoshino and kirishima. Yoshino especially is a breath of fresh air as a protag. I didn't expect her to actually
I keep worrying they're going to try to redeem this guy, but it seems less and less like that's in the cards, lol. My prediction is that each and every person who appears "normal" will either become or be revealed to already be unhinged, and I am here for it.