Gelmar/Geruma is a devil with power over water. He is sent by Sirene to attack Miki. In truth, he is fodder sent by Sirene to weaken him so that she may finish him off with ease. 

Demons exist and their power is beyond what humans can fathom. In fact, they are so strong that humans do not stand a chance against them in a fight. 
The only thing strong enough to defeat a demon is another demon, and it is through this logic that Ryo Asuka hatches the plan to have his good-hearted friend, Akira Fudo, possessed by a demon. If a person is of pure of heart, then they might be able to control the demon that possesses them, and thereby acquire power equal to a demon. After raising some hell in a club, the plan works, and Akira is possessed by the powerful demon known as Amon. 
Now Akira is humanity's sole champion of justice against the hidden demon menace that has plagued humanity since the dawn of time.
 
(Source: Anime News Network)

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Hordes of demons, once thought entombed forever in the underworld, have now returned to Earth. Evil and corruption begin to seep into our world, as demons seize human hosts. Mankind’s only hope for salvation is to use the demons’ power against them–and only a pure-hearted man like Fudo Akira can do so without losing his humanity. Once little more than a crybaby, Akira now wields the terrible power of a devil, yet holds the innocent soul of a man–Devilman! 
 
(Source: Seven Seas Entertainment)

New Devilman project marking Go Nagai's 50th anniversary as a creator. The protagonist Akira Fudou learns from his best friend, Ryou Asuka, that an ancient race of demons has returned to take back the world from humans. Ryou tells Akira that the only way to defeat the demons is to incorporate their supernatural powers, and suggests that he unite with a demon himself. Akira succeeds in transforming into Devilman, who possesses both the powers of a demon and the soul of a human. The battle of Devilman and Akira Fudou begins. (Source: Netflix)

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