My favorite rarepair is HifumixGundham because of how sweet and extra they are when they geek out over their nerd shit. Like they're so enthusiastic that Kiibo gets fully into hearing about the anime villain backstory they invent for him. And Gundham has obviously read Hifumi's doujin because he knows about the nsfw ones.
They sneak off together all the time to read manga. Gundham is always 100% in the cliche shounen stuff with 8500 panels of powering up before a battle while Hifumi reads his moe Piggles shit but whenever they hit a cool scene they swap volumes so the other one can read it too.
Oh YES finally someone mentioned them!
Hifumi bounces his ideas off of Gundham and works out the kinks in his stories. They have the most intense conversations about manga and art and which characters could beat who. Forget what I said the other night, THEY call their friends and discuss these things and their friends try to keep up. Years later if you bring up The Great Piggles Debate they'll immediately start up the argument again, but it's all in jest.
When they're not talking about that, Gundham is rambling about animals. He loves showing Hifumi the animals he's currently working with, and sometimes Hifumi ends up taking inspiration from them! Gundham also has trouble with physical touch, so when he lets Hifumi hold his hand for the first time it's An Event™.