Ooh I love this pairing! So nice~ I also wasn’t sure what to do, so I just wrote some headcanony things, hope it’s okay!
The most important thing in this relationship is respecting each other’s boundaries. Since Dark Shadow is very tame when it’s light, Tokoyami trusts his s/o not to abuse their power. (Judging by how dark his room is, I think the darkness is important for him)
But his s/o is also perfect to help him calm down, or if Dark Shadow is a bit too out of hand, they can help! He loves having someone who helps him have a good grasp of things.
He can’t give up being edgy I’m sorry that’s just the way he is. His s/o can’t make fun of him of course, but he also won’t expect them to adapt to his lifestyle.
Like the request says, ying and yang! Live and coexist together in harmony, but neither conforms to the other, either.
Whenever his s/o needs to intimidate someone, Tokoyami will gladly do so for them!