-When Deidara and Hidan first interacted, Deidara hated Hidan. Hidan is immortal, and fights by surviving things that should kill him, and Deidara sees this as the worst kind of heresy according to his philosophy. For a long time, Deidara loathed and was disgusted by Hidan.
-Eventually things came to a head, Deidara going overboard when expressing his disgust of Hidan (who has a short fuse in the first place) and they had a vicious fistfight which involved them rolling around in the grass, very close to each other, in a way quite unbecoming of S-class wanted-nin.
-Hidan ended up with several obvious bitemarks in…unfortunate places. Kakuzu made plenty of suggestive remarks to the effect of what Hidan and Deidara might have been doing, much to Hidan’s frustration. Deidara, for his part, would keep pointedly taking out breath mints from his robes.
-The most important thing that came from their encounter, however, Hidan snarling “Do you think I fucking like this?!” and Deidara realizing that Hidan was more trapped in an immortal body than enjoying it (as opposed to Sasori, who sought immortality)
-And I mean Deidara in general isn’t the most sympathetic person, but staying immortal seems to him like a living hell on earth and he honestly feels pity for Hidan
-That ended their rivalry
-They do have similar personalities, and occasionally take a personal “vacations,” mostly involving explosions and slaughter. It’s their main form of bonding.
-Deidara once let Hidan do his hair. Once was enough.
-Deidara helped Hidan make a lot of modifications to his scythe - making it extend and retract, split into different, smaller parts, etc. He used the same mechanical expertise he employed for his mechanical eye.
-Hidan has inconsistent relationship patterns - very passionate one second and withdrawn the next, his impulsive personality and the knowledge that he would survive while Deidara died, and not wanting to get attached.
-Deidara, of course, lives purely in the moment and gives everything his all.
-As they spent more time together, they began to occasionally exchange speech patterns - Deidara swearing more, and Hidan sometimes punctuating his statements with a grunt. Sasori is the only one who notices, but he doesn’t say anything.
-After Hidan disappears, Deidara spends a very long time searching for him before finally concluding he’s dead
-That’s around the time he begins to unwind, becoming a little more unstable (Sasori might have been able to help, but Sasori is dead) - he doesn’t let himself mourn for Hidan, because he knows that it’s what Hidan would have wanted.
-He does, however, make a few sacrifices and a few prayers to Jashin as best he can, in memory of Hidan’s soul. He intended, in part, to make Sasuke the last
- because Jashin is all about shared pain, and that would be realized if they both died in the same explosion.