I have a question about the mercenary copilot. Does his ability only work on the initial dice you roll, or can it be used on the modified dice?

For example I roll at range 3 Focus Focus Blank, and spend a focus to get hit hit blank. Can I then use the mercenary to make one of those hits a crit? Or can I not because there were no hits rolled initially?

The FAQ addresses this.

Q: If a player has multiple effects that resolve at the same time, can he resolve them in any order?

A: Yes.

Because choosing to use a focus token or Merc Copilot resolves at the same time, you can choose the order that is best for you.

That's what I thought, thanks!