If I use Charm to move an enemy unit with Assault onto Ahri's battlefield, does that enemy unit count as the attacker and get Assault? Also, when does Ahri's Nine-Tailed Fox ability get added to the chain?

Ruling: Yes, the enemy unit is the attacker because the attacker is the player whose unit creates the contested state at the battlefield. The player who controls the unit that moved is the player who applied contested status.

Sequence: (for Nine-Tailed Fox trigger timing) Case A - Attacker is the turn player:

• Attacker's "When I attack" triggers
• Nine-Tailed Fox trigger
• Defender's "When I defend" triggers

Case B - Defender is first in turn order (e.g., via Charm):

• Nine-Tailed Fox trigger
• Attacker's "When I attack" triggers
• Defender's "When I defend" triggers

Nuances:

• Nine-Tailed Fox is treated as an attack trigger ("when X attacks"), not a defend trigger, even though it's controlled by the defender
• When multiple attack triggers are controlled by different players, turn order determines the sequence
• Attack triggers always resolve before defend triggers