When attacking, if a unit is in a battlefield and Yasuo enters, can Retreat be used to avoid the damage from Yasuo's ability?

Ruling: Yes, you can play Retreat as a Reaction to Yasuo's ability on the Chain to avoid the damage.

Sequence:

• Yasuo enters the battlefield and his ability triggers
• The ability goes on the Chain
• You can play Retreat as a Reaction
• Your unit leaves the battlefield
• Yasuo's ability misses because the targeted unit is no longer there

Nuances:

• Even if Yasuo's ability targets your unit, "here" refers to the battlefield Yasuo is at, so if your unit isn't there anymore when the ability resolves, it misses