Popcorn is Midnite Matinee's trusty AdArm Pack-model light AutoHollow.
Popcorn is a humanoid mech shaped like a jackal person. At about 20 feet tall, she's bigger than Thisbe but smaller than the Hippocampus. Her level of sapience is about on par with Mow's.[1] She's equipped with a single machine gun,[2] and as an AutoHollow, she can go into "Titanfall-style auto mode."[3]
In another parallel to Thisbe and Mow, Midnite at least sometimes rides Popcorn on the outside rather than sitting in a cockpit.[4]