We know that he has a Mega Blaziken and we know that he is mysterious and he is probably important for the plot. Here is some guesses:
1. He is a rival
They may want to have Megas introduced by a rival and wants this guy to be a mystery guy?
He feels like a guy who can dress up and walk around with ubermons.
3. The champion
Why not? A mysterious champion would make a funny plot.
4. A guy from Team Flare
They seems to be Fire type fans, so a Mega Blaziken would fit. And this can be a good way of introducing them.
5. A random guy named Ernie
Just think of it! The best option.