Every hero can perform a Melee attack to quickly finish of an enemy or do damage while reloading. The damage scales with levels and with certain Items.
A melee attack can be charged up to also dash forward and do more damage. Charging the melee attack can be done in the air.
Last hitting a Trooper with a melee attack will not make an orb appear and give you the full amount of souls you would have gotten if you shot the creep and the orb.