Diamrem is the Unfortunate Mermaid. As one of the "unfortunate monsters", she is a fish with human legs. Like a fish, she seems to be unable to breathe outside of water, as implied when she occasionally says "W...Water..."
She first appears along with Amira and her other Unfortunate Friends in Succubus Village, after Goddess Ilias declares war on the world. Once Luka defeats Eva, the mastermind behind the attack, but is unable to stop the self-sustaining Great Barrier of Babylon that continues to put the human men into a trance and make them ejaculate constantly, Amira and her Unfortunate Friends arrive and their odd, non-sexual appearance cures the men of their trances and eradicates the barrier. The villagers then thank the unfortunate monsters, though they are concerned for the unfortunate mermaid and ask her if she should return to the ocean.
One month after Luka defeats Ilias, Luka visits the Succubus Village to check on the unfortunates. When she is seen, she is floating in the sky and speaks in a single ellipse. Amira claims that she "became a star during that final battle" and that her "soul is still burning brightly inside their stomachs." Pyhar seems confused, but comes to the conclusion that the sushi Amira gave the Unfortunate Friends as a reward was actually Diamrem.