If Irresistible Faefolk moves to an empty battlefield and brings an enemy unit, who is the attacker?

When you move Irresistible Faefolk to an unoccupied (uncontrolled) battlefield and use its ability to move an enemy unit to that same battlefield, you are the attacker and the enemy unit is the defender.

According to RiftJudge FAQ #9828, #9714, and Rule 459.2.b.1, the determination of attacker and defender designations is based on which player applied the "Contested" status to the battlefield first.

Sequence of Events

1. Move: You move Irresistible Faefolk to an unoccupied battlefield. This applies the "Contested" status to that battlefield. Because you are the player who applied this status, you gain the Attacker designation.
2. Trigger: Irresistible Faefolk's ability ("When I move to a battlefield...") triggers. You place this ability on the chain.
3. Resolution: When the ability resolves, you move an enemy unit to that same battlefield.
4. Combat Transition: Because units from opposing players are now present at the contested battlefield, the showdown transitions into a Combat Showdown.

Even though your ability moved the enemy unit, you remain the attacker because you were the one who initiated the combat by applying the "Contested" status to the battlefield first.