The Runner has a greenish color, A lanky build with elongated limbs, and runs at the player. If you spot one on a wall, take note that they do not begin chase unless you open the crates, get too close, or shoot them.
A good strategy to deal with Runners is to use the rifle's charged shot. They tend to strafe and dodge often, so melee or shoot one to stun it, then fire a charged shot. Runners are affected by flares and other distracting equipment. Another way to deal with them is by lighting them on fire.
The Runner can be first seen on the first level.