Gargant is an important antagonist in the Onimusha series. Although he is important to the plot, he doesn't appear as a boss in any game. Like Marcellus, Gargant is an Oni warrior turned genma demon.
He worked his way up the demon ladder and serves as Nobunaga's right hand man, and was one of the greatest Genma warriors during his time. He is a good friend of fellow demon swordsman Gogandantess, and like him has a strong sense of honor and respect for his opponents.
Gargant served as a stalwart Oni warrior who fought many battles against the Genma demons. However, by his own will, he betrayed the Oni and went to serve the Genma and served Fortinbras. After Fortinbras' death, he became Nobunaga's most trusted warrior and fought in Nobunaga's conquests.
Onimusha: Blade Warriors
Garant makes his first appearance in Onimusha: Blade Warriors. Little is revealed about what his story entails, but it is known that he fought against the Oni warriors, particularly Samanosuke. Over a decade later, while Japan is in a constant state of civil war. Gargant manages to kill Samanosuke's close friend and personal kunoichi, Kaede, in a battle. Samanosuke swears revenge.
Onimusha 3: Demon Siege
In a cinema before the game starts, Gargant is shown leading a huge army to conquer parts of Japan. Samanosuke is fighting on the opposing side. Samanosuke fights through the entire Genma army and manages to fight Gargant in a duel. After a grueling fight, Samanosuke changes into Onimusha (an extremely powerful Oni form) and unleashes a potent shockwave destroying Gargant.
Onimusha: Dawn of Dreams
In the fourth Onimusha game, Gargant appears revived and fights the protagonist. When the protagonist wins, he is granted great power for his victory.