I'm not sure that ni (H tone) should be called a definite article. It does, however, single out a nominal that has already been introduced in the discourse (and presumably known in the context), as with i in Gbaya. However, because of what I consider its excessive use I think that ni is not being used rigorously in the old way. I suspect that change is taking place under the influence of French. On the other hand, I should think that more research on the use of ni today would teach me a lot.
Source: nd
Source: nd