He is portrayed by Justice Leak.
The Hellgrammite is described as an insect capable of taking any shape it wants. As it's body is not carbon based but depending on chlorine, it is forced to break into chemical facilities to find DDT, the material closest to its food source.
The Hellgrammite was arrested by Krypton's military guild, brought before Alura Zor-El, and sentenced to Fort Rozz. After Fort Rozz's crash to Earth, the Hellgrammite escaped along with the rest of its fellow convicts.
On earth, the Hellgrammite is at first seen at Plastino Chemicals, tampering with a vat of chemicals. It is found by a security guard who calls backup after the Hellgrammite can't show any ID. When the guard draws his gun, the Hellgrammite dislodges its jaw and screams at the guard, causing the terrified man to run away. The Hellgrammite swiftly catches up to the guard, jumps onto him and kills him, leaving the body behind.
It is later confronted by two of General Astra's men and Astra herself. Astra reveals that the Hellgrammite will make perfect bait for Kara and orders the Hellgrammite to kill her. After finding out about the DDT, the DEO sets a trap for the Hellgrammite by driving a truck full of DDT about the highway. Indeed the Hellgrammite soon smells the DDT and jumps onto the truck. When the DEO agents start shooting at the Hellgrammite out of their car, the Hellgrammite lunges stingers drawn from its own body at the agents, jumps onto their car and grabs Alex, a DEO agent and Kara's foster sister. It delivers Alex to Astra, hoping that this would mean that Astra would leave it alone.
While Astra fought against Supergirl, the Hellgrammite tried to kill Alex with one of its stingers but Alex dodged the blow and stabbed the Hellgrammite into the heart with its own stinger, killing it.