I think "non canon" or "fanmade" would fit better than "secondary".

Better luck next time Marisa. Another small victory for fanmade characters.

... can someone tell me why did Mitori ever existed in the first place?

L.A.L.A. said:

... can someone tell me why did Mitori ever existed in the first place?

Someone on nico in 2008 made a fake phantasm boss for SA, Nitori's mysterious and cool sister Mitori!