Osamu Dazai.

A cold breeze blew across the roof of the Armed Detective Agency, but his calm and impeccable demeanor remained unwavering. Since his time in the Port Mafia, where he worked alongside his best friend Chuuya, his work had been perfect. Sloppy footsteps announced Dazai's arrival, sporting the same sly smile he'd known since adolescence, before they switched to the Agency.
— Ah, my dear… Admiring the horizon all alone? — Dazai leaned in with a dramatically passionate look. — The sun in your eyes makes this the perfect moment for a poetic double suicide. What do you say to taking that step with me today?
"Be quiet, Dazai," you replied without altering your serene tone, taking a sip of your tea.
He laughed, used to rejection. Dazai loved to provoke, but he respected her strength. As Akutagawa's older sister, her ability was an ancient and superior version of Rashomon: the Black Chant (Kuro no Uta), capable of shaping shadows into devouring mists and intangible armor.
Ryunosuke appeared on the terrace. Any other man with such witticisms would be destroyed by him, but desperately seeking Dazai's approval, the youngest allowed the former executive to get close.
"Sister. The mission reports have been delivered," Ryunosuke said, until a dry cough broke his breath. He covered his mouth with his handkerchief.
With a motherly gesture, you covered his chest with the mist of your breathing-easing skill, hating to see how your brother was choking, seeking validation from Osamu.
"You've worked so hard, Ryunosuke. You don't need to prove anything to anyone," you murmured affectionately.
Dazai watched the scene. Behind the playful banter, he admired his unconditional protectiveness towards his youngest son.
"She's right, Akutagawa," Dazai said, touching the young man's shoulder, who froze. Dazai then winked at you: "It would be a shame if my future brother-in-law fell ill before you accepted my marriage proposal!"
— Dazai… be quiet — you repeated calmly, maintaining a slight and almost imperceptible smile.