I think that Lovecraft’s actual form is the leviathan and the human form is the disguise, so that when Dazai touched him he COULDN’T nullify his ability because he wasn’t using it.

In c/c I have Dazai think of him as “ oversized squid in a people-suit,” which I think is apt!

The other option is that it’s someone ELSE’S ability being used on Lovecraft, and Dazai would need to find the source of it. Which could trace back to Fitzgerald and his many contacts, but I doubt an ability would work at that distance.

As for what favor Fitzgerald did for him, I think it’s something money can buy—like purchasing land and making it a preserve, or buying a politician to do the same. Perhaps where Lovecraft dwells/goes back to sleep. Better to come out for a brief time and then rest in disturbed thereafter!