Mori, but he cares for Dazai in a way that is most certainly toxic as he tries to help. He gives Dazai vitamin medication and vitamin shots pretty much all the time, and gives him check ups twice a week. But Dazai doesn't want to be helped, so Mori reverts to underhanded tactics like restraining him and sedating him so that way Dazai can be healthy.

Mori, at least once a month, becomes convinced Dazai has some sort of disease or some sort of physical problem, leading to more testing on Dazai's end. They both have more than enough anxiety over it, for very different reasons, but it ends up being fine.

Mori, as the good father he is, is very concerned for Dazai's physical health. He still needs Dazai out on the front lines, but that's what he is there for, to help Dazai heal and get better. If he's healthy, he's happy. Sure Dazai refuses to eat most anything, but that's why Mori's there. Sure Dazai has the worst sleep schedule, but that's why Mori's there. Sure Dazai is still trying to kill himself, but that's why Mori is there. All he has to do is lie indefinitely about giving him "a painless suicide", and he'll stay by his side forever. And Mori can make sure he stays alive.

He'll survive, but only under Mori's care.

The mafia is the place for them.