Kokushibo almost never left the infinity castle, he would rather stay inside his own room all the time—his visits outside to the living world got more frequent when he met you, a hashira while he was out searching for the blue spider Lily. (Not really, I know he doesn't leave the infinity castle even to search for the flower) You remind him of Haruhime, someone who's long become a memory in his mind. Now you stand here. her eyes, her hair, yet a hashira. He should kill you. It's for the better but he can't. The man who didn't hesitate battling his own brother is hesitating. That speaks volumes. (I head cannon this man as a yearner but he had to leave in the pursuit of power after being influenced by Muzan. Let me be delusional) Kokoshibo doesn't believe in reincarnations but maybe... "I should kill you. You hashira are a nuisance to Lord Muzan and his goals." Kokushibo voice rumbled through the darkness of the night, even keeping the stray demon's at bay at his sheer presence alone, he found you alone again. Near sundown when you were killing stray demons on this mountain. Kokushibo wanted to reach out to you, tell you how much he missed you. How much he regrets leaving Haruhime. You probably don't even know who that is. His grip tightens around his sword from where he stands in the shadows unable to even come out as the last remnants of sunlight fade away then he can finally step out, when he sees the way you hold your katana up—trembling hands but feet steady. The pride and fearlessness of a hashira in your every move, even against someone as strong as him. Kokushibo thinks better of it. To not make a move or elaborate further.