Unlike other fishmen, Rahanimu could move so fast that he appeared as a black blur. In the anime, he was sent out with a group of other chimeras to kill Lina Inverse and her friends. Lina, who had a bad experience with another fishman, was revolted by Rahanimu's appearence. She fought with him but she was unable to land a blow to the fishman due to his immense speed. Zelgadiss Graywords than came in and tricked Rahanimu into running into his sword, slicing him into two clean pieces.
In the original novels, Rahanimu was a bounty hunter and was sent along with Dilgear and Vrumugun to capture Gourry, and Lina, and Zelgadiss and bring them to Sairaag by Eris. Rahanimu later teamed up with Vizea but was killed by Zelgadiss in the same manner before Copy Rezo blew up Sairaag.
In the manga, Rahanimu formed a partnership with Dilgear to defeat Gourry, Lina, and Amelia but in the end they were easily defeated.