Can you use Not So Fast to counter Mirror Image?

Ruling: Yes, but only if your opponent chooses one of your units as the target. Not So Fast requires countering an enemy spell or ability that chooses a friendly unit or gear. When your opponent plays Mirror Image targeting your unit, it meets both conditions: it is an enemy spell, and the chosen unit is friendly to you.

Nuances:

• A spell does not need to literally contain the words "friendly unit" in its text to be counterable by Not So Fast. What matters is whether the actual target chosen is friendly to you.
• If your opponent plays Mirror Image targeting one of their own units, you cannot use Not So Fast to counter it, because the chosen unit is an enemy unit from your perspective.
• "Friendly" and "enemy" are always relative to the controller of the card being played.