If I use Charm to move an enemy unit to a battlefield I control, am I the attacker or defender?

Ruling: You are the defender. The attacker is determined by which player controls the units that applied the contested status to the battlefield, not by who played the spell that moved the units.

Nuances:

• The current rules could be read as "the player who played the spell is the attacker," but this interpretation is incorrect
• A future Comprehensive Rules update will clarify that the attacker is the controller of the units that applied contested status
• If you control a battlefield and use Charm to bring an enemy unit there, the enemy unit applies the contested status, making your opponent the attacker and you the defender