Been thinking more about basilisk!Dazai and human!Chuuya lately and will probably write something eventually so here I go with more headcanons:
Dazai having an affinity for warm baths and soaking in the tub inhaling the steam. He'd love going to the beach and walking along the shore as well. He'd also really like rolling around in the warm sand and getting his feet wet by the water.
He's down to do anything by the beach because the sand and the sea salt on the wind feels so nice to him so they often have date nights on the beach making little campfires and roasting marshmallows. Also, volleyball...building sandcastles...trying to surf...I say trying because I don't think Dazai has very good sea legs when he's bipedal. I think he'd have coordination issues—falling into the water a lot (and doesn't even mind it)
When he's not in his human form, he sheds his skin much like a snake and the cycle reflects in his human form: after shedding, in his human form his skin is silky smooth, hair fluffier, his eyes crystal clear, and he doesn't wear as many bandages to cover his skin because the scales recede. But the longer the cycle goes on up until it's time to shed again, his skin grows rougher (and he wears a lot more bandages), there's a cloudiness in his eyes, and he's sensitive to light and sounds and just wants to curl up in bed.
Chuuya going out to get medicine at the pharmacy for Dazai thinking he's ill but Dazai's gone when Chuuya returns and there's a note by the bed that 'something came up' and Dazai needed to leave, aka his shitty excuse to go back into the sea to actually shed his skin because it's agony sustaining his human form.
Chuuya finds out about the truth either when Dazai's acting really weird and locking himself in a room and refusing to come out because he lost a contact lenses and doesn't want to kill Chuuya, or something really mundane like Dazai gets something in his eye, it makes his eye water, loosens the contact lenses, and some poor person just happened to be looking at him and was petrified on the spot
Chuuya feeling the subtle fluctuations in Dazai's body temperature. Sometimes Dazai's a little cooler without the lights on or the heat going. Or in the winter. He'd love being bundled in heating blankets during the winter. Chuuya turning on a little heating lamp on the nightstand and Dazai just kinda scooting closer and snuggling next to it.
Chuuya sometimes hearing on the news about the Tokyo Bay's waters being unusually turbulent that time of year and the sea life being restless in response to Dazai's fluctuating emotions. No one thinks it's an ancient creature at hand, just unusual weather patterns. Stressed Dazai just putting everything around him on edge—fishermen have a hard time leaving the ports, and more wildlife is seen beached on the shores.
Dazai brings a piece of treasure from the depths to subtly test Chuuya to see if he's actually pure hearted or might be just faking it and waiting to submit to human greed. Dazai's seen it happen countless times so when Chuuya's in awe and doesn't even want to touch it because he feels like he might disturb some ancient treasure, Dazai's heart swells a lil bit and makes him more curious about Chuuya.
Chuuya starts having vivid dreams of the sea more often the longer they're together; he's like seeing through Dazai's eyes in his dreams.
When Chuuya does put all the pieces together and realizes what Dazai is after extensive research, he goes to the shore one late night when it's silent and there's no activity to talk to the water and try to coax Dazai into resurfacing when he thinks Dazai's either not there, ignoring him, or maybe Chuuya had it wrong all along, and Chuuya just talks to the ocean in the hopes his voice carries and Dazai hears. Dazai just hearing Chuuya say that not all humans are bad. And that Chuuya really loved him from the bottom of his heart. That he's human, so he can't stick around forever, but that he forgives Dazai and he knows loneliness too.
Dazai doesn't resurface immediately because he's not in his human form and doesn't want to harm Chuuya, but the sky clears, the constant rain lets up, the ocean calms, and Chuuya hears about it on the news how it's the first beautiful day in a while in Yokohama and it's a metaphor for Dazai finding peace
