Hot off the heels of what was a fantastic (but some-what flawed) Season Finale, I've been thinking about the Oni, in particular, their presence in Ninjago. Mistaké's reveal was surprising, and it got me thinking, were there others like her. How many existing characters have we've seen that are secretly Oni...?
According to evidence found in the last two seasons, the Oni travelled to Ninjago to attempt to bring the First Spinjitzu Master back to the first realm. (We can assumed that the Overlord could've been involved in one of those attempts, but that's besides the point).
They failed. The FSM stayed and then subsequently died in Ninjago.
So they found the next best solution, to bring Wu and Garmadon back. How? By starting a war that would force them to flee Ninjago, for the safety of them and they're people.
The Serpentine War
It's a bit of a stretch, but I believe that Master Chen is an Oni...
His quirky and strange attributes are not unlike Mistaké's, and him training Garmadon seems significantly more relevant.