The manga only speaks of two worlds - demon world representing dark, and human world representing light. There is no third one as far as we were told, so going by what Arkham said, angels and demons are one and the same specie, possibly simply of different allegiance (we know struggles for throne are not uncommon in demon world), which would be similar to our world's mythology that says demons are fallen angels.
Since Arius was researching into ruler of demon world that preceded Mundus, there is a possibility he also researched into angels, thinking of them as demons. Perhaps the reason why Chi was proclaimed defect was that she exhibited properties of the angel part of her genetic code, whereas Secretaries were demons.
Until Capcom explicitly says otherwise, she is a demon. An artificially created demon with a Devil Trigger. Speculation is just speculation.