Can you counter Charm with Not So Fast?

Ruling: Yes, you can counter Charm with Not So Fast. When your opponent plays Charm targeting your unit, it is an enemy spell that chooses a friendly unit, meeting both requirements of Not So Fast.

Nuances:

• Charm targets the unit it moves, making it a valid target for Not So Fast.
• The key is perspective: when your opponent's Charm targets your unit, that unit is "friendly" from your point of view, satisfying Not So Fast's requirement.