The strength of warriors past will revive you in battle.
Demongo was an enemy of Samurai Jack, and is one of Aku's most powerful servants. Demongo is known as "The Soul Stealer" or "The Soul Collector." Though he looks tough, he has a squeaky voice.
Demongo appears within Huntor's Crest in the Darklands in the past. Though he normally has strange powers, he tells the player during a DexterGuide Mission that Fusion Demongo stole them. Demongo seems to know about a super-powerful weapon that could be created from the remains of several of Fuse's enemies that had gone berserk briefly beforehand. He tells the player that spirits of ancient warriors attack Fusion monsters, using their powers to turn them into weapons upon defeat. Demongo also provides information about Aku in Dexter's final Guide Mission. He doesn't reveal whether or not he is working for Aku, but he says that Aku is near and that he is watching.
Fusion Demongo is apparently one of Fuse's most trusted fusions, since he appears to fight alongside Lord Fuse when the player invades his castle.
Just as Demongo was the right-hand man of Aku, Fusion Demongo is the right-hand man of Fuse.