Gromos was a minor character in the first episode of Season 5. He's a Demon who possessed the body of recently deceased killer, Vernon Gill, but the body was too damaged to keep working for long. Gromos, had been sent from Hell by Lucifer, to deliver a message to Chloe Decker.
History
Gromos is a demon who resides in Hell with his siblings, Dromos, Squee, Belios, and the other demons. When a murder suspect, Vernon Gill, was killed by being struck by a car while running from Detective Decker, he went to Hell, and with a clear vacancy available, the demon Gromos possessed his recently deceased body, having been sent by Lucifer to deliver a lead to Chloe.
When Gromos arrived, he spewed up some blood and apologized to the startled Chloe for being "gross." He then delivered his message, and realized the body he was in was "too jacked up" to keep functioning, and just before he departed his vessel, he recognized his sibling, Maze, standing nearby, "Is that Mazikeen?" He then returned to Hell, leaving the body behind.
Personality
Gromos is a loyal demon, devoted to Lucifer Morningstar, and willing to do his King's bidding.