When you use Charm on an opponent's unit and bring it to your occupied battlefield, who is the attacker and defender?

Ruling: When you Charm an opponent's unit to a battlefield you control, your opponent becomes the attacker and you are the defender. The attacker is the player who gave the contested status to the battlefield.

Nuances:

• The charmed unit's controller (your opponent) is the attacker, so they get any "when attacking" effects
• The attacker gains focus and priority first
• You can use Charm to move units between battlefields regardless of whether they have ganking; the restriction on inter-battlefield movement only applies to the Standard Move action