Sirene was a relatively friendly rival to Mera and a love interest to Aquaman.
Sirene comes from an alien race with shape-shifting abilities. She has an amphibian physiology and hydrokinetic (water control) powers similar to those of Mera. Her brother Rovere was involved in a gang. When Sirene convinced him to quit it, they discovered it was not that easy. The other gang members were set after the "traitor". The siblings had to flee through space. Arriving on Earth, the duo took to its oceans. Rovere captured Aquaman and assumed his form and role.
What they had not expected was a jealous Mera to confront them. Mera thought Aquaman had dumped her for an alien girl. She tried to use her hard-water powers to stop her former lover from escaping. Sirene however countered Mera's every move with one of her own. Mera seemed to be quitting her efforts and leaving in defeat. Actually she silently followed the new "couple", seeking a better opportunity. She observed as "Aquaman" turned back to Rovere's regular form.
Mera ambushed Sirene and threatened her to change Aquaman back to normal. Sirene had to explain their story and the location of the actual Aquaman. Mera went to free her lover. But the duo were soon confronted by the alien gang. The gang members had "recognized" Mera as Sirene, believing the hydrokinetic powers to be unique. Aqualad had already been captured. Sirene and Rovere then appeared, having shape-shifted into giant crabs. They soon freed the three captives and escaped with them.
Sirene revealed a personal reason for helping them. She was romantically interested in Arthur. Prompting a new confrontation between her and Mera (often described as a "catfight"). While thus distracted, none of the two ladies noticed Rovere turning himself in to the gang. He had tired of others being endangered while he fled. Mera and Sirene had just enough time to transfer Arthur and Garth to the gang's ship as it left.
Later the two Atlanteans returned with Rovere and the hijacked ship. Sirene reluctantly agreed to leave, but only after a goodbye kiss with her love interest.