Maybe dark isn't the word I was looking for but I just mean the way they interact with each other.
For example, Reina always says Kumiko has a "bad personality" (meaning it In a good way), they sometimes give each other "payback" for what the other said, and just the way they generally interact is awesome.
I guess it's not dark at a all since they don't do anything bad, its just a result of their two personality types together.