Demon was once a Seraphimon, but he fell and became Demon. He also was once able to obtain a secret form called Super Final, but he was devolved and unable to re-obtain it - but he still had a plan. He created a Super Final egg, which hatched into Arkadimon, who COULD evolve to Super Final - and when the time was right, he let Arkadimon delete him. When Arkadimon had evolved to Super Final, Demon was able to take control of his body and become Demon Super Final once more.
Etemonkey the Etemon
Etemonkey was the second in command of Demon's army, until Neo came along. Etemonkey only toyed with Taichi and Zero, for he enjoyed watching their battles. When Neo arrived and killed Etemonkey, Etemonkey revealed that all he really wanted to do, the reason he joined Demon's army, was in the hopes of going to the human world and playing with human children. However, Demon's plan was to conquer the human world! But Neo did not care, and he deleted Etemonkey.