Yumiko is a Shooter hero, with the following Leader Trait:

Whenever Yumiko attacks an enemy, all ranged allies within range of the target will attack it as well for a portion of their damage.
As a leader, every ranged ally gains this ability.

Weapons' areas of attack apply normally. For example, an assault survivor with a wide arc weapon will also damage other enemies that would normally be hit with a regular attack.

Note: The trait does not apply if Yumiko's target dies