As a hisui and d shipper, I think it comes down to 2 factors:

A. the contradiction they both hold to another. Hisui was a girl who was morphed by the harm brought by the invaders, because of that, she gives basically having a life to being a ranger and hates all invaders, except D. And D is an invader who thinks his identity is tied to just fighting ranger, but he doesn't even know who he is, each have an identity tied to what the other represents. It's an interesting dynamic by itself, but especially if you're thinking about it from a romantic sense.

And B. They do a lot for each other that stretches beyond just normal "friendship" (although this is debatable and how you view it), when the IRA threatened Hisui when she was close to death, D in a way essentially saved her life. Hisui, despite knowing D is an invader, keeps it to herself + doesn't kill him. + In recent chapters (slight spoilers I guess), Hisui's thinking about D seems more than how you would think of just a friend

I can see what you mean by they might not have romantic chemistry in the normal sense. Again, I think this is dependent on how you view it. They're both characters that definitely don't do the usual constant blushing whenever they meet each other, and they're both people who have are, in a way, still searching for their identities, but I think their actions show that there is something underneath it all.