Green Devil was the alias used by Vel Koon during the early years of his life. He was given the nickname through his involvement in a wide variety of Arena Tournaments, and it was given to him because of the green-colored armor he always wore. Green Devil was reported to be killed by Eldrick Kuraine after his take over of Loronar Security. In a sense, Green Devil did die, as it was a name that Vel Koon never used again.
See Vel Koon for more details.