It’s true that I use quite a few characters with some form of dual personality. To keep it simple, it’s mostly a coincidence rather than something directly tied to the story itself — these are just characters that are popular within the community.
That said, characters with dual identities or contrasting sides are always interesting to explore. You can see that with Marika — without going too deep into spoilers, she can shift drastically from one extreme to another, and that’s partly because of her dual nature. It also suggests that one side of her might be more relevant than the other depending on how the overall story progresses.