In the second volume of the manga and in the OVA Devilman: The Demon Bird, Ghelmer was summoned by Sirene when she attempted to kill Devilman. Infiltrating the Makimura household, he merged with the water in the bathtub while Akira's girlfriend Miki was bathing. Sensing demons nearby, Akira burst into the bathroom to find Miki trapped within a vortex with Ghelmer looming over her. Akira attacked Ghelmer and forced him to release Miki, but Miki had swallowed some of the water that Ghelmer had melded with, allowing the demon to control her. Akira was forced to knock Miki out to break Ghelmer's control over her and then cast a bedsheet over the monster, setting it on fire and evaporating the water Ghelmer had fused with, causing him to solidify. Vulnerable in his solid form, Ghelmer was unable to fight back as Devilman lunged at him and proceeded to rip him apart.
Ghelmer is hydrokinetic and can control any body of water he comes into contact with. He can meld with the water and can even control persons that swallow any liquids that are part of him, as demonstrated when he possessed Miki during his fight with Devilman.
Ghelmer is vulnerable to intense heat and evaporating his water form will cause him to become solid. He lacks any conventional fighting ability and is entirely reliant on water for his powers.