I think … Kakuzu, in his time with Hidan, grew to care about him. Whether romantically or as a friend or simply a teammate is up to interpretation. But I think Hidan filled up some of the empty space in Kakuzu’s heart(s), and maybe gave him some comfort that when everything was over, he wouldn’t have to spend his (perceived) immortality by himself. Now, I don’t think it’s in Hidan’s nature to be “down”, or, rather, to SHOW that he’s “down” about anything. I think his religion promotes toughness and brutality and any show of emotion other than your love for Lord Jashin is considered a flaw, a disgusting weakness. So for Hidan to SHOW that he’s down, whether intentionally or not, would be a rarity, and likely a cause of concern for Kakuzu. But Kakuzu’s very unused to being sympathetic or kind or exhibiting any of those human traits, so I don’t know if he’d properly know how to comfort Hidan. I could see him possibly attempting to make Hidan’s favorite meal and leaving it outside his door ((the thought never occurs to him that Hidan might enjoy eating *together*)). Or possibly a small pat on the back ((which he would have to be careful of because I think Hidan’s immediate reaction to unexpected physical contact would be to lash out)). Or maybe, just a simple “Are you okay?”. Sometimes that helps tremendously, just having someone take the time to look you in the eye, acknowledge that something may be wrong, and inquire about your well-being. Might open the floodgates for a rare (but probably needed) conversation between the two, maybe one where Hidan admits his insecurities about his religion, or being in the Akatsuki, or anything at all, and Kakuzu tells him he has the same feelings. Maybe. 🤷🏽♀️